diff --git a/api/lib/canonical-url.ts b/api/lib/canonical-url.ts
index 2f7c4246..00b567c2 100644
--- a/api/lib/canonical-url.ts
+++ b/api/lib/canonical-url.ts
@@ -13,6 +13,8 @@
  * @module
  */
 
+import { siteHostOf } from "../../shared/url.ts";
+
 const TRACKING_PARAMS = new Set([
   "fbclid",
   "gclid",
@@ -52,7 +54,7 @@ const YOUTUBE_HOSTS = new Set([
  * uses it too, so both agree on what a YouTube link is.
  */
 export function youtubeHost(u: URL): string | null {
-  const host = u.hostname.toLowerCase().replace(/^www\./, "");
+  const host = siteHostOf(u);
   return YOUTUBE_HOSTS.has(host) ? host : null;
 }
 
@@ -90,7 +92,7 @@ export function canonicalizeUrl(raw: string): string | null {
   }
   if (u.protocol !== "http:" && u.protocol !== "https:") return null;
 
-  const host = u.hostname.toLowerCase().replace(/^www\./, "");
+  const host = siteHostOf(u);
   if (!host) return null;
 
   const videoId = youtubeVideoId(host, u.pathname, u.searchParams);
diff --git a/api/lib/css-color.ts b/api/lib/css-color.ts
new file mode 100644
index 00000000..dc81fa44
--- /dev/null
+++ b/api/lib/css-color.ts
@@ -0,0 +1,91 @@
+/**
+ * Reads the CSS colors the server meets: a dumped page's `theme-color`, which
+ * the generic link preview keeps as its tint (`normalizeCssColor`), and the
+ * theme stylesheets' tokens, which the emails resolve (`theme-tokens.ts`).
+ *
+ * @module
+ */
+
+/** An sRGB color with its alpha, each channel 0–255 and alpha 0–1. */
+export interface Rgba {
+  r: number;
+  g: number;
+  b: number;
+  a: number;
+}
+
+/**
+ * The named colors sites actually put in `theme-color`, and `transparent`,
+ * which the themes write; the rest of CSS's names aren't worth carrying.
+ */
+const NAMED: Record<string, string> = {
+  transparent: "#00000000",
+  black: "#000000",
+  white: "#ffffff",
+  red: "#ff0000",
+  green: "#008000",
+  blue: "#0000ff",
+  yellow: "#ffff00",
+  orange: "#ffa500",
+  purple: "#800080",
+  gray: "#808080",
+  grey: "#808080",
+  silver: "#c0c0c0",
+  maroon: "#800000",
+  navy: "#000080",
+  teal: "#008080",
+  olive: "#808000",
+  lime: "#00ff00",
+  aqua: "#00ffff",
+  cyan: "#00ffff",
+  fuchsia: "#ff00ff",
+  magenta: "#ff00ff",
+};
+
+/** Reads a channel of `rgb()`, a number or a percentage of 255. */
+function channel(text: string): number {
+  const n = parseFloat(text);
+  const value = text.endsWith("%") ? (n / 100) * 255 : n;
+  return Math.max(0, Math.min(255, value));
+}
+
+/** Reads the alpha of `rgba()`, a number or a percentage of 1. */
+function alpha(text: string): number {
+  const n = parseFloat(text);
+  return Math.max(0, Math.min(1, text.endsWith("%") ? n / 100 : n));
+}
+
+/**
+ * Returns the color `text` writes as a hex (3, 4, 6 or 8 digits), an `rgb()`
+ * or `rgba()` of numbers or percentages, or one of the common names; undefined
+ * for any other form. Channels out of range are clamped.
+ */
+export function parseCssColor(text: string): Rgba | undefined {
+  const value = text.trim().toLowerCase();
+  const hex = /^#([0-9a-f]{3,8})$/.exec(NAMED[value] ?? value)?.[1];
+  if (hex) {
+    if (hex.length === 5 || hex.length === 7) return undefined;
+    const full = hex.length <= 4 ? [...hex].map((c) => c + c).join("") : hex;
+    const byte = (i: number) => parseInt(full.slice(i * 2, i * 2 + 2), 16);
+    return {
+      r: byte(0),
+      g: byte(1),
+      b: byte(2),
+      a: full.length === 8 ? byte(3) / 255 : 1,
+    };
+  }
+  const args = /^rgba?\(([^)]+)\)$/.exec(value)?.[1];
+  if (!args) return undefined;
+  const parts = args.split(/[\s,/]+/).filter(Boolean);
+  if (parts.length !== 3 && parts.length !== 4) return undefined;
+  const [r, g, b] = parts.slice(0, 3).map(channel);
+  const a = parts[3] === undefined ? 1 : alpha(parts[3]);
+  return [r, g, b, a].some(Number.isNaN) ? undefined : { r, g, b, a };
+}
+
+/** Writes `color` as a lowercase `#rrggbb`, its alpha dropped. */
+export function hexOf({ r, g, b }: Rgba): string {
+  return `#${
+    [r, g, b].map((c) => Math.round(c).toString(16).padStart(2, "0")).join("")
+  }`;
+}
diff --git a/api/lib/slugify.ts b/api/lib/slugify.ts
index 592c6a82..ce98cdaf 100644
--- a/api/lib/slugify.ts
+++ b/api/lib/slugify.ts
@@ -72,14 +72,15 @@ export function uniqueSlug(
 
 /**
  * Throws a 400 unless `slug` is a valid slug, lowercase words of letters and
- * digits joined by hyphens, at most `SLUG_MAX` long. Dump and playlist slugs
- * reserve no word: nothing else a user names lives beside them.
+ * digits joined by hyphens, at most `max` long. slug-service checks a dump's
+ * or a playlist's with it, and category-service a category's, with that
+ * kind's own limit.
  */
-export function validateSlug(slug: string): void {
-  if (!isValidSlug(slug)) {
+export function validateSlug(slug: string, max: number = SLUG_MAX): void {
+  if (!isValidSlug(slug, max)) {
     throw new APIException(
       400,
-      `Slug must be lowercase letters, numbers and hyphens, at most ${SLUG_MAX} characters`,
+      `Slug must be lowercase letters, numbers and hyphens, at most ${max} characters`,
     );
   }
 }
diff --git a/api/lib/ttl-cache.ts b/api/lib/ttl-cache.ts
new file mode 100644
index 00000000..e9d720f5
--- /dev/null
+++ b/api/lib/ttl-cache.ts
@@ -0,0 +1,60 @@
+/**
+ * Keeps what the server fetched from another site for a while, by key, so a
+ * page or a feed asking again doesn't refetch it: Bandcamp tracklists, YouTube
+ * playlists, and link previews. Each holder sets how long an entry lives and
+ * how many are kept; past that count, the oldest written goes first.
+ *
+ * @module
+ */
+
+/**
+ * A map whose entries expire `ttlMs` after they are written, holding
+ * `maxEntries` at most.
+ *
+ * @example
+ * const playlists = new TtlCache<YouTubePlaylist>(6 * 3_600_000, 200);
+ * const hit = playlists.get(listId) ?? playlists.set(listId, await fetch());
+ */
+export class TtlCache<V> {
+  #entries = new Map<string, { value: V; at: number }>();
+
+  constructor(readonly ttlMs: number, readonly maxEntries: number) {}
+
+  /** Returns the value under `key` unless it is missing or expired. */
+  get(key: string): V | undefined {
+    const hit = this.#entries.get(key);
+    if (!hit) return undefined;
+    if (Date.now() - hit.at >= this.ttlMs) {
+      this.#entries.delete(key);
+      return undefined;
+    }
+    return hit.value;
+  }
+
+  /** Stores `value` under `key` from now on, and returns it. */
+  set(key: string, value: V): V {
+    this.#entries.delete(key);
+    // A Map keeps insertion order, so the first key is the oldest write.
+    if (this.#entries.size >= this.maxEntries) {
+      const oldest = this.#entries.keys().next().value;
+      if (oldest !== undefined) this.#entries.delete(oldest);
+    }
+    this.#entries.set(key, { value, at: Date.now() });
+    return value;
+  }
+
+  /**
+   * Removes `key`, or only when `value` is still what it holds, so a late
+   * failure can't remove the entry that replaced its own.
+   */
+  delete(key: string, value?: V): void {
+    if (value === undefined || this.#entries.get(key)?.value === value) {
+      this.#entries.delete(key);
+    }
+  }
+
+  /** Empties the cache, for the tests' reset between cases. */
+  clear(): void {
+    this.#entries.clear();
+  }
+}
diff --git a/api/mail/theme-tokens.ts b/api/mail/theme-tokens.ts
index 9690cccf..a2259ec3 100644
--- a/api/mail/theme-tokens.ts
+++ b/api/mail/theme-tokens.ts
@@ -16,18 +16,11 @@
  */
 
 import { BASE_STYLE, type StyleName } from "../../shared/theme.ts";
+import { hexOf, parseCssColor, type Rgba } from "../lib/css-color.ts";
 
 /** A scheme a token is resolved for. "auto" is drawn from both. */
 export type Scheme = "light" | "dark";
 
-/** An sRGB color with its alpha, each channel 0–255 and alpha 0–1. */
-interface Rgba {
-  r: number;
-  g: number;
-  b: number;
-  a: number;
-}
-
 /**
  * Returned by {@link readTokens}: the tokens of one style in one scheme, read
  * as the kind of value each is.
@@ -134,33 +127,16 @@ function replaceCalls(
 
 /** Parses a CSS color in the forms the themes write, or throws. */
 function parseColor(text: string): Rgba {
-  const value = text.trim().toLowerCase();
-  if (value === "transparent") return { r: 0, g: 0, b: 0, a: 0 };
-  const hex = value.match(/^#([0-9a-f]{3,8})$/)?.[1];
-  if (hex && hex.length !== 5 && hex.length !== 7) {
-    const full = hex.length <= 4 ? [...hex].map((c) => c + c).join("") : hex;
-    const channel = (i: number) => parseInt(full.slice(i * 2, i * 2 + 2), 16);
-    return {
-      r: channel(0),
-      g: channel(1),
-      b: channel(2),
-      a: full.length === 8 ? channel(3) / 255 : 1,
-    };
-  }
-  const rgb = value.match(/^rgba?\((.*)\)$/)?.[1];
-  if (rgb) {
-    const [r, g, b, a = "1"] = rgb.split(/[\s,/]+/).filter(Boolean);
-    const alpha = a.endsWith("%") ? parseFloat(a) / 100 : parseFloat(a);
-    return { r: +r, g: +g, b: +b, a: alpha };
-  }
-  throw new Error(`Unreadable color: ${text}`);
+  const color = parseCssColor(text);
+  if (!color) throw new Error(`Unreadable color: ${text}`);
+  return color;
 }
 
 /** Writes `color` as `#rrggbb`, or as `rgba()` when it is translucent. */
-function formatColor({ r, g, b, a }: Rgba): string {
-  const [R, G, B] = [r, g, b].map((c) => Math.round(c));
-  if (a < 1) return `rgba(${R},${G},${B},${+a.toFixed(3)})`;
-  return `#${[R, G, B].map((c) => c.toString(16).padStart(2, "0")).join("")}`;
+function formatColor(color: Rgba): string {
+  if (color.a >= 1) return hexOf(color);
+  const [r, g, b] = [color.r, color.g, color.b].map(Math.round);
+  return `rgba(${r},${g},${b},${+color.a.toFixed(3)})`;
 }
 
 /** Returns `top` laid over `under`, both straight (not premultiplied) alpha. */
diff --git a/api/remote/bandcamp-streams.ts b/api/remote/bandcamp-streams.ts
index 94a1e171..af867482 100644
--- a/api/remote/bandcamp-streams.ts
+++ b/api/remote/bandcamp-streams.ts
@@ -9,19 +9,16 @@
  */
 
 import { APIException } from "../lib/api-exception.ts";
+import { TtlCache } from "../lib/ttl-cache.ts";
 import { fetchWithTimeout, isValidHttpUrl, readTextCapped } from "./fetch.ts";
 import { BANDCAMP_HOST_RE, parseTralbum } from "./bandcamp-tralbum.ts";
 import type { Tralbum } from "../../shared/player.ts";
 
 /**
- * Half the signatures' life, so a cached URL still has at least 12 hours left
- * when it is handed out.
+ * Lives half the signatures' life, so a cached URL still has at least 12
+ * hours left when it is handed out.
  */
-const TTL_MS = 12 * 60 * 60 * 1000;
-/** The oldest write goes first. */
-const MAX_ENTRIES = 200;
-
-const cache = new Map<string, Tralbum>();
+const cache = new TtlCache<Tralbum>(12 * 3_600_000, 200);
 
 /**
  * Returns `url` without its query, fragment and trailing slashes, and with its
@@ -38,25 +35,6 @@ function cacheKey(url: string): string {
   }
 }
 
-function readCache(key: string): Tralbum | null {
-  const hit = cache.get(key);
-  if (!hit) return null;
-  if (Date.now() - hit.resolvedAt > TTL_MS) {
-    cache.delete(key);
-    return null;
-  }
-  return hit;
-}
-
-function writeCache(key: string, value: Tralbum): void {
-  // Map preserves insertion order, so the first key is the oldest write.
-  if (cache.size >= MAX_ENTRIES) {
-    const oldest = cache.keys().next().value;
-    if (oldest !== undefined) cache.delete(oldest);
-  }
-  cache.set(key, value);
-}
-
 /**
  * Returns the tracklist of the Bandcamp page `pageUrl`, from the cache when it
  * was resolved in the last 12 hours. `force` skips the cache: the player sets
@@ -87,7 +65,7 @@ export async function resolveBandcamp(
 
   const key = cacheKey(pageUrl);
   if (!force) {
-    const hit = readCache(key);
+    const hit = cache.get(key);
     if (hit) return hit;
   }
 
@@ -115,6 +93,5 @@ export async function resolveBandcamp(
     throw new APIException(404, "No streamable tracks on this page");
   }
 
-  writeCache(key, parsed);
-  return parsed;
+  return cache.set(key, parsed);
 }
diff --git a/api/remote/fetch.ts b/api/remote/fetch.ts
index a72f1884..6109f778 100644
--- a/api/remote/fetch.ts
+++ b/api/remote/fetch.ts
@@ -7,7 +7,11 @@
  * @module
  */
 
-import { isPrivateHostname, resolvePublicAddress } from "./public-address.ts";
+import {
+  bareHostname,
+  isPrivateHostname,
+  resolvePublicAddress,
+} from "./public-address.ts";
 import { isSelfUrl } from "../lib/self-url.ts";
 
 const FETCH_HEADERS = {
@@ -43,7 +47,7 @@ async function fetchViaCurl(
         "--proto",
         "=http,https",
         "--resolve",
-        `${url.hostname.replace(/^\[(.*)\]$/, "$1")}:${port}:${pinned}`,
+        `${bareHostname(url)}:${port}:${pinned}`,
         "--max-time",
         String(Math.ceil(timeoutMs / 1000)),
         // curl fails on a larger file rather than stopping at a cap, so this is
diff --git a/api/remote/html.ts b/api/remote/html.ts
index ea7f38ec..57e2f9d6 100644
--- a/api/remote/html.ts
+++ b/api/remote/html.ts
@@ -7,6 +7,8 @@
  * @module
  */
 
+import { hexOf, parseCssColor } from "../lib/css-color.ts";
+
 /**
  * Decodes the five XML entities and numeric ones in `str`. `&amp;` goes first,
  * so `&amp;quot;` decodes twice; `bandcamp-tralbum.ts`, which decodes JSON,
@@ -118,74 +120,15 @@ export function extractMaskIconColor(html: string): string | undefined {
   return undefined;
 }
 
-/**
- * The named colors sites actually put in `theme-color`; the rest of CSS's names
- * aren't worth carrying.
- */
-const NAMED_COLORS: Record<string, string> = {
-  black: "#000000",
-  white: "#ffffff",
-  red: "#ff0000",
-  green: "#008000",
-  blue: "#0000ff",
-  yellow: "#ffff00",
-  orange: "#ffa500",
-  purple: "#800080",
-  gray: "#808080",
-  grey: "#808080",
-  silver: "#c0c0c0",
-  maroon: "#800000",
-  navy: "#000080",
-  teal: "#008080",
-  olive: "#808000",
-  lime: "#00ff00",
-  aqua: "#00ffff",
-  cyan: "#00ffff",
-  fuchsia: "#ff00ff",
-  magenta: "#ff00ff",
-};
-
-function clampByte(n: number): number {
-  return Math.max(0, Math.min(255, Math.round(n)));
-}
-
 /**
  * Returns `raw` as a lowercase `#rrggbb`, from a hex, `rgb()`, `rgba()` or
  * common named color; undefined for any other form. The generic provider
  * stores it as the preview's `accentColor`, a tint over the app's own surface,
- * so alpha is dropped.
+ * so alpha is dropped, and a fully transparent one is no color at all.
  */
 export function normalizeCssColor(raw: string | undefined): string | undefined {
-  if (!raw) return undefined;
-  const value = raw.trim().toLowerCase();
-
-  const named = NAMED_COLORS[value];
-  if (named) return named;
-
-  const hex = /^#([0-9a-f]{3,8})$/.exec(value)?.[1];
-  if (hex) {
-    if (hex.length === 3 || hex.length === 4) {
-      const [r, g, b] = [...hex.slice(0, 3)];
-      return `#${r}${r}${g}${g}${b}${b}`;
-    }
-    if (hex.length === 6 || hex.length === 8) return `#${hex.slice(0, 6)}`;
-    return undefined;
-  }
-
-  const rgb = /^rgba?\(([^)]+)\)$/.exec(value)?.[1];
-  if (rgb) {
-    const parts = rgb.split(/[\s,/]+/).filter(Boolean).slice(0, 3);
-    if (parts.length !== 3) return undefined;
-    const bytes = parts.map((p) => {
-      const n = parseFloat(p);
-      if (!Number.isFinite(n)) return NaN;
-      return clampByte(p.endsWith("%") ? (n / 100) * 255 : n);
-    });
-    if (bytes.some(Number.isNaN)) return undefined;
-    return `#${bytes.map((b) => b.toString(16).padStart(2, "0")).join("")}`;
-  }
-
-  return undefined;
+  const color = raw ? parseCssColor(raw) : undefined;
+  return color && color.a > 0 ? hexOf(color) : undefined;
 }
 
 /**
diff --git a/api/remote/providers/generic.ts b/api/remote/providers/generic.ts
index e1a7a309..c3b58c38 100644
--- a/api/remote/providers/generic.ts
+++ b/api/remote/providers/generic.ts
@@ -7,6 +7,7 @@
 
 import type { RichContent } from "../../../shared/dump.ts";
 import type { RichContentProvider } from "./provider.ts";
+import { siteHost } from "../../../shared/url.ts";
 import {
   extractBestIcon,
   extractFirstContentImage,
@@ -66,7 +67,7 @@ export const genericProvider: RichContentProvider = {
       extractPageTitle(html);
 
     const siteName = extractOgTag(html, "site_name") ??
-      new URL(url).hostname.replace(/^www\./, "");
+      siteHost(url);
 
     const description = extractOgTag(html, "description") ??
       extractMetaName(html, "twitter:description") ??
diff --git a/api/remote/providers/youtube.ts b/api/remote/providers/youtube.ts
index 8780cb28..45d16945 100644
--- a/api/remote/providers/youtube.ts
+++ b/api/remote/providers/youtube.ts
@@ -11,12 +11,13 @@ import type { RichContentProvider } from "./provider.ts";
 import { extractOgTag } from "../html.ts";
 import { fetchWithTimeout, readTextCapped } from "../fetch.ts";
 import { youtubeHost, youtubeVideoId } from "../../lib/canonical-url.ts";
+import { siteHostOf } from "../../../shared/url.ts";
 
 /** Returns the id of the video `url` points at, in any shape YouTube uses. */
 function extractVideoId(url: string): string | null {
   try {
     const u = new URL(url);
-    const host = u.hostname.toLowerCase().replace(/^www\./, "");
+    const host = siteHostOf(u);
     return youtubeVideoId(host, u.pathname, u.searchParams);
   } catch {
     return null;
diff --git a/api/remote/public-address.ts b/api/remote/public-address.ts
index 096c417f..ac71afe3 100644
--- a/api/remote/public-address.ts
+++ b/api/remote/public-address.ts
@@ -100,9 +100,10 @@ function isIpLiteral(host: string): boolean {
 
 /**
  * Returns `url`'s hostname without the brackets `URL.hostname` keeps around
- * an IPv6 literal.
+ * an IPv6 literal: the address checks here read it, and `fetch.ts` hands it
+ * to curl's `--resolve`.
  */
-function bareHostname(url: URL): string {
+export function bareHostname(url: URL): string {
   return url.hostname.replace(/^\[(.*)\]$/, "$1");
 }
 
diff --git a/api/remote/youtube-playlist.ts b/api/remote/youtube-playlist.ts
index 1599720f..331b2df2 100644
--- a/api/remote/youtube-playlist.ts
+++ b/api/remote/youtube-playlist.ts
@@ -13,6 +13,7 @@
  */
 
 import { APIException } from "../lib/api-exception.ts";
+import { TtlCache } from "../lib/ttl-cache.ts";
 import { readTextCapped } from "./fetch.ts";
 import type {
   YouTubePlaylist,
@@ -20,11 +21,7 @@ import type {
 } from "../../shared/player.ts";
 
 /** Titles and lengths rarely change, and a stale one only mislabels a row. */
-const TTL_MS = 6 * 60 * 60 * 1000;
-/** The oldest write goes first. */
-const MAX_ENTRIES = 200;
-
-const cache = new Map<string, { value: YouTubePlaylist; at: number }>();
+const cache = new TtlCache<YouTubePlaylist>(6 * 3_600_000, 200);
 
 /**
  * Admits every kind of list id (`PL…`, `OLAK5uy_…`, `RD…`) and nothing else.
@@ -167,8 +164,7 @@ export async function resolveYouTubePlaylist(
   }
 
   const hit = cache.get(listId);
-  if (hit && Date.now() - hit.at < TTL_MS) return hit.value;
-  if (hit) cache.delete(listId);
+  if (hit) return hit;
 
   const controller = new AbortController();
   const timer = setTimeout(() => controller.abort(), 8000);
@@ -198,10 +194,5 @@ export async function resolveYouTubePlaylist(
     throw new APIException(404, "No tracks on this playlist");
   }
 
-  if (cache.size >= MAX_ENTRIES) {
-    const oldest = cache.keys().next().value;
-    if (oldest !== undefined) cache.delete(oldest);
-  }
-  cache.set(listId, { value: parsed, at: Date.now() });
-  return parsed;
+  return cache.set(listId, parsed);
 }
diff --git a/api/routes/attachments.ts b/api/routes/attachments.ts
index 714115f4..fc14dd28 100644
--- a/api/routes/attachments.ts
+++ b/api/routes/attachments.ts
@@ -20,7 +20,7 @@ import {
 import { authenticateMedia } from "../lib/auth.ts";
 import { type APIResponse } from "../../shared/api.ts";
 import { APIException } from "../lib/api-exception.ts";
-import type { AttachmentResponse } from "../../shared/dump.ts";
+import type { AttachmentResponse } from "../../shared/attachment.ts";
 import { serveUploadedFile, validateImageUpload } from "../lib/upload.ts";
 import { ATTACHMENTS_DIR } from "../config.ts";
 
diff --git a/api/routes/files.ts b/api/routes/files.ts
index a24d6c73..037f2a7c 100644
--- a/api/routes/files.ts
+++ b/api/routes/files.ts
@@ -16,18 +16,15 @@
 import { Router } from "@oak/oak";
 import { APIException } from "../lib/api-exception.ts";
 import type { APIResponse } from "../../shared/api.ts";
-import type { Dump } from "../../shared/dump.ts";
+import { type Dump, dumpFileVersion } from "../../shared/dump.ts";
 import { DUMPS_DIR } from "../config.ts";
+import { UUID_RE } from "../../shared/validation.ts";
 import { authenticateMedia } from "../lib/auth.ts";
 import { can } from "../lib/permissions.ts";
 import { serveFile } from "../lib/serve-file.ts";
 import { authMiddleware } from "../middleware/auth.ts";
 import { parseCategoryIds } from "../validators/dump.ts";
-import {
-  dumpFileVersion,
-  getDump,
-  replaceFileDump,
-} from "../services/dump-service.ts";
+import { getDump, replaceFileDump } from "../services/dump-service.ts";
 
 /**
  * Mounted by `api/main.ts` behind the error, Open Graph and CORS middleware.
@@ -71,7 +68,7 @@ router.get("/:dumpId/file", async (ctx) => {
   const { dumpId } = ctx.params;
 
   // The id names a file on disk: nothing but an id's characters gets through.
-  if (!/^[0-9a-f-]{36}$/.test(dumpId)) {
+  if (!UUID_RE.test(dumpId)) {
     throw new APIException(400, "Invalid dump ID");
   }
 
diff --git a/api/routes/thumbnails.ts b/api/routes/thumbnails.ts
index e0b38e91..ac7870f6 100644
--- a/api/routes/thumbnails.ts
+++ b/api/routes/thumbnails.ts
@@ -22,9 +22,8 @@
 import { Router } from "@oak/oak";
 import { APIException } from "../lib/api-exception.ts";
 import type { APIResponse } from "../../shared/api.ts";
-import type { Dump } from "../../shared/dump.ts";
+import { type Dump, dumpFileVersion } from "../../shared/dump.ts";
 import {
-  dumpFileVersion,
   getDump,
   getDumpThumbnailInfo,
   removeDumpThumbnail,
@@ -37,6 +36,7 @@ import {
   validateImageUpload,
 } from "../lib/upload.ts";
 import { DUMPS_DIR, THUMBNAILS_DIR } from "../config.ts";
+import { UUID_RE } from "../../shared/validation.ts";
 import { authenticateMedia } from "../lib/auth.ts";
 import { can } from "../lib/permissions.ts";
 import { type AuthContext, authMiddleware } from "../middleware/auth.ts";
@@ -190,7 +190,7 @@ router.get("/:dumpId/thumbnail", async (ctx) => {
   const { dumpId } = ctx.params;
 
   // The id names files on disk: nothing but an id's characters gets through.
-  if (!/^[0-9a-f-]{36}$/.test(dumpId)) {
+  if (!UUID_RE.test(dumpId)) {
     throw new APIException(400, "Invalid dump ID");
   }
 
diff --git a/api/routes/ws.ts b/api/routes/ws.ts
index bf5facda..f812859c 100644
--- a/api/routes/ws.ts
+++ b/api/routes/ws.ts
@@ -27,6 +27,7 @@ import {
   isUserChatOpen,
   type RateLimit,
   register,
+  send,
   sendToUser,
   setClientTyping,
   takeToken,
@@ -59,10 +60,7 @@ import {
 } from "../services/comment-like-service.ts";
 import { getUserById } from "../services/user-service.ts";
 import { APIException } from "../lib/api-exception.ts";
-import type {
-  ClientToServerMessage,
-  ServerToClientMessage,
-} from "../../shared/ws.ts";
+import type { ClientToServerMessage } from "../../shared/ws.ts";
 import type { User } from "../model/user.ts";
 
 /**
@@ -125,7 +123,7 @@ router.get("/ws", async (ctx) => {
       const myCommentLikes = user ? getUserCommentLikes(user.id) : [];
       const unreadNotificationCount = user ? getUnreadCount(user.id) : 0;
       const chatRead = user ? getChatReadState(user.id) : undefined;
-      socket.send(JSON.stringify({
+      send(socket, {
         type: "welcome",
         users: getOnlineUsers(),
         myVotes,
@@ -133,7 +131,7 @@ router.get("/ws", async (ctx) => {
         unreadNotificationCount,
         chatReadAt: chatRead?.readAt,
         unreadChatCount: chatRead?.unread ?? 0,
-      }));
+      });
     } catch (err) {
       console.error("[ws] welcome send failed:", err);
     }
@@ -223,11 +221,6 @@ function handleMessage(client: WsClient, data: unknown): void {
   }
 }
 
-/** Sends `data` unless the socket is closing, where `send` would throw. */
-function send(socket: WebSocket, data: ServerToClientMessage): void {
-  if (socket.readyState === WebSocket.OPEN) socket.send(JSON.stringify(data));
-}
-
 /**
  * Handles `vote_cast` and `vote_remove`. It acks the voter with `vote_ack` and
  * broadcasts `votes_update`. A failure goes back as an `error` naming the dump
diff --git a/api/services/attachment-service.ts b/api/services/attachment-service.ts
index 415b9348..af8eefb4 100644
--- a/api/services/attachment-service.ts
+++ b/api/services/attachment-service.ts
@@ -9,6 +9,7 @@
 
 import { db, transaction } from "../db/connection.ts";
 import { APIException } from "../lib/api-exception.ts";
+import { attachmentIdsIn } from "../../shared/attachment.ts";
 
 /**
  * Records attachment `id` once `POST /api/attachments` has written its file.
@@ -108,9 +109,6 @@ function resourceOwner(
   }
 }
 
-const ATTACHMENT_RE =
-  /\/api\/attachments\/([0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12})/g;
-
 /**
  * Binds the attachments whose URLs appear in `text` to the resource that saved
  * it, so the startup purge keeps them as long as that resource exists, and
@@ -124,7 +122,7 @@ export function linkAttachments(
   resourceType: AttachmentResourceType,
   resourceId: string,
 ): void {
-  const ids = [...text.matchAll(ATTACHMENT_RE)].map((m) => m[1]);
+  const ids = attachmentIdsIn(text);
   if (ids.length === 0) return;
   const update = db.prepare(
     "UPDATE attachments SET resource_type = ?, resource_id = ? WHERE id = ? AND resource_id IS NULL",
diff --git a/api/services/category-service.ts b/api/services/category-service.ts
index e6608de3..729e2bdd 100644
--- a/api/services/category-service.ts
+++ b/api/services/category-service.ts
@@ -22,27 +22,17 @@ import {
 } from "../model/dump.ts";
 import { type Page, pageOf } from "../lib/pagination.ts";
 import { isReservedSlug } from "../lib/reserved-slugs.ts";
-import { SLUG_RE } from "../../shared/slug.ts";
+import { validateSlug } from "../lib/slugify.ts";
 import { VALIDATION } from "../../shared/validation.ts";
 import { parseRows } from "../model/rows.ts";
 
 /**
- * Throws a 400 unless `slug` is well-formed, short enough and not reserved,
- * with a message for each case.
+ * Throws a 400 unless `slug` is a valid category slug: a slug within a
+ * category's length, and none of the top-level paths other pages hold
+ * (reserved-slugs.ts), since a category's feed lives at the URL root.
  */
-function validateSlug(slug: string): void {
-  if (!SLUG_RE.test(slug)) {
-    throw new APIException(
-      400,
-      "Slug must be lowercase letters, numbers and hyphens",
-    );
-  }
-  if (slug.length > VALIDATION.CATEGORY_SLUG_MAX) {
-    throw new APIException(
-      400,
-      `Slug must be at most ${VALIDATION.CATEGORY_SLUG_MAX} characters`,
-    );
-  }
+function validateCategorySlug(slug: string): void {
+  validateSlug(slug, VALIDATION.CATEGORY_SLUG_MAX);
   if (isReservedSlug(slug)) {
     throw new APIException(400, `Slug "${slug}" is reserved`);
   }
@@ -133,7 +123,7 @@ export function listCategories(): Category[] {
  * either is taken.
  */
 export function createCategory(req: CreateCategoryRequest): Category {
-  validateSlug(req.slug);
+  validateCategorySlug(req.slug);
   const name = requireName(req.name);
 
   const id = crypto.randomUUID();
@@ -177,7 +167,7 @@ export function updateCategory(
 
   const newSlug = req.slug ?? existing.slug;
   if (req.slug !== undefined && req.slug !== existing.slug) {
-    validateSlug(req.slug);
+    validateCategorySlug(req.slug);
   }
   const newName = req.name !== undefined
     ? requireName(req.name)
diff --git a/api/services/digest-service.ts b/api/services/digest-service.ts
index 3f10391d..1e98fc1e 100644
--- a/api/services/digest-service.ts
+++ b/api/services/digest-service.ts
@@ -24,7 +24,9 @@ import {
   PERIOD_MS,
 } from "../../shared/settings.ts";
 import type { CommunityStats } from "../../shared/community.ts";
-import type { Dump } from "../../shared/dump.ts";
+import { type Dump, dumpFileVersion } from "../../shared/dump.ts";
+import { siteHost } from "../../shared/url.ts";
+import { markdownToPlainText } from "../../shared/markdown.ts";
 import {
   DUMP_SELECT_COLUMNS_ALIASED,
   dumpRowToApi,
@@ -61,7 +63,6 @@ import { lookOf } from "../mail/layout.ts";
 import { createUnsubscribeToken } from "../lib/jwt.ts";
 import { isEmailEnabled, sendEmail } from "./email-service.ts";
 import { getSettings } from "./settings-service.ts";
-import { dumpFileVersion } from "./dump-service.ts";
 import { withCurrentRefs } from "./notification-service.ts";
 import { communityStatsOf } from "./community-service.ts";
 
@@ -93,12 +94,7 @@ interface Window {
 /** Flattens Markdown into a short line of text for a quote. */
 function quoteOf(markdown: string | undefined | null): string | undefined {
   if (!markdown) return undefined;
-  const flat = markdown
-    .replace(/!\[[^\]]*\]\([^)]*\)/g, "")
-    .replace(/\[([^\]]*)\]\([^)]*\)/g, "$1")
-    .replace(/[`*_>#~]/g, "")
-    .replace(/\s+/g, " ")
-    .trim();
+  const flat = markdownToPlainText(markdown, "");
   if (!flat) return undefined;
   return flat.length > QUOTE_MAX
     ? `${flat.slice(0, QUOTE_MAX - 1).trimEnd()}…`
@@ -131,14 +127,8 @@ function thumbnailOf(dump: Dump): string | null {
 }
 
 function toDigestDump(dump: Dump, readerId: string): DigestDump {
-  let domain: string | null = null;
-  if (dump.kind === "url" && dump.url) {
-    try {
-      domain = new URL(dump.url).hostname.replace(/^www\./, "");
-    } catch {
-      domain = null;
-    }
-  }
+  const domain = (dump.kind === "url" && dump.url && siteHost(dump.url)) ||
+    null;
   return {
     title: dump.title,
     url: `${FRONTEND_URL}${dumpPath(dump)}`,
diff --git a/api/services/dump-service.ts b/api/services/dump-service.ts
index 1bb5f798..55b3ff85 100644
--- a/api/services/dump-service.ts
+++ b/api/services/dump-service.ts
@@ -17,6 +17,7 @@ import type {
   DumpUrlMatch,
   UpdateDumpRequest,
 } from "../../shared/dump.ts";
+import { siteHost } from "../../shared/url.ts";
 import { db, transaction } from "../db/connection.ts";
 import {
   DUMP_SELECT_COLUMNS as SELECT_COLS,
@@ -83,15 +84,6 @@ function assertFileDumpText(title?: string, comment?: string): void {
   }
 }
 
-/** Returns a link's host name, the title it gets when its page has none. */
-function titleFromUrl(url: string): string {
-  try {
-    return new URL(url).hostname.replace(/^www\./, "");
-  } catch {
-    return url;
-  }
-}
-
 /**
  * Notifies the followers of a dump that went public, `before` being private
  * and `after` public: the poster's, as if it were just posted, and those of
@@ -129,7 +121,7 @@ export async function createUrlDump(
   // A title typed on the create form wins over the one scraped from the page:
   // the poster has seen the preview and is correcting it.
   const title = request.title?.trim() || richContent?.title ||
-    titleFromUrl(request.url);
+    (siteHost(request.url) ?? request.url);
   const isPrivate = request.isPrivate ?? false;
 
   const categoryIds = [...new Set(request.categoryIds ?? [])];
@@ -244,20 +236,6 @@ export async function createFileDump(
   return dump;
 }
 
-/**
- * Returns a file dump's version, which the app's file and thumbnail URLs carry
- * as `?v=`. It changes whenever the file may have, so the file and thumbnail
- * routes cache a URL naming the current one for good. The frontend's
- * `dumpFileVersion` must compute the same.
- */
-export function dumpFileVersion(
-  dump: Pick<Dump, "fileSize" | "createdAt" | "updatedAt">,
-): string {
-  return `${dump.fileSize ?? 0}-${
-    (dump.updatedAt ?? dump.createdAt).getTime()
-  }`;
-}
-
 /**
  * Deletes the thumbnails that `routes/thumbnails.ts` generated from a dump's
  * upload, once the upload is replaced or deleted. The route makes them again
@@ -542,7 +520,7 @@ export async function updateDump(
 
   if (newUrl !== dump.url) {
     richContent = await fetchRichContent(newUrl);
-    title = richContent?.title ?? titleFromUrl(newUrl);
+    title = richContent?.title ?? siteHost(newUrl) ?? newUrl;
   }
 
   if (request.title?.trim()) {
@@ -819,7 +797,7 @@ export async function refreshDumpMetadata(dumpId: string): Promise<Dump> {
     );
   }
 
-  const title = richContent?.title ?? titleFromUrl(dump.url);
+  const title = richContent?.title ?? siteHost(dump.url) ?? dump.url;
 
   const updatedDump: Dump = { ...dump, title, richContent };
   const row = dumpApiToRow(updatedDump);
diff --git a/api/services/rich-content-service.ts b/api/services/rich-content-service.ts
index 64db6f8a..3ea1629d 100644
--- a/api/services/rich-content-service.ts
+++ b/api/services/rich-content-service.ts
@@ -8,6 +8,8 @@
  */
 
 import type { RichContent } from "../../shared/dump.ts";
+import { siteHost } from "../../shared/url.ts";
+import { TtlCache } from "../lib/ttl-cache.ts";
 import type { RichContentProvider } from "../remote/providers/provider.ts";
 import { youtubeProvider } from "../remote/providers/youtube.ts";
 import { bandcampProvider } from "../remote/providers/bandcamp.ts";
@@ -47,19 +49,12 @@ export interface FetchRichContentResult {
   content?: RichContent;
 }
 
-/** How long a fetched preview is reused. */
-const RECENT_TTL_MS = 5 * 60 * 1000;
-/** How many previews are kept. The oldest written goes first. */
-const RECENT_MAX = 200;
 /**
  * Makes posting a link fast after the create form previewed it, by keeping the
- * previews fetched in the last few minutes, by URL. Without it, the second
+ * previews fetched in the last five minutes, by URL. Without it, the second
  * fetch of the page was the slow part of posting. Only successes are kept.
  */
-const recent = new Map<
-  string,
-  { at: number; result: Promise<FetchRichContentResult> }
->();
+const recent = new TtlCache<Promise<FetchRichContentResult>>(5 * 60_000, 200);
 
 /**
  * Forgets every cached preview. The tests' `stubNetwork` calls it, since
@@ -78,20 +73,13 @@ export function tryFetchRichContent(
   url: string,
   { fresh = false }: { fresh?: boolean } = {},
 ): Promise<FetchRichContentResult> {
-  const hit = recent.get(url);
-  if (!fresh && hit && Date.now() - hit.at < RECENT_TTL_MS) return hit.result;
-  recent.delete(url);
+  const hit = fresh ? undefined : recent.get(url);
+  if (hit) return hit;
 
   // Stored while in flight too, so a preview and a post racing share a fetch.
-  const result = fetchRichContentUncached(url);
-  if (recent.size >= RECENT_MAX) {
-    const oldest = recent.keys().next().value;
-    if (oldest !== undefined) recent.delete(oldest);
-  }
-  const entry = { at: Date.now(), result };
-  recent.set(url, entry);
+  const result = recent.set(url, fetchRichContentUncached(url));
   result.then(({ ok }) => {
-    if (!ok && recent.get(url) === entry) recent.delete(url);
+    if (!ok) recent.delete(url, result);
   });
   return result;
 }
@@ -108,18 +96,10 @@ async function fetchRichContentUncached(
     return { ok: true, content: await provider.fetch(url) };
   } catch (err) {
     console.error(`[rich-content] Failed to fetch metadata for ${url}:`, err);
-    try {
-      return {
-        ok: false,
-        content: {
-          type: "generic",
-          url,
-          siteName: new URL(url).hostname.replace(/^www\./, ""),
-        },
-      };
-    } catch {
-      return { ok: false };
-    }
+    const siteName = siteHost(url);
+    return siteName
+      ? { ok: false, content: { type: "generic", url, siteName } }
+      : { ok: false };
   }
 }
 
diff --git a/api/services/ws-service.ts b/api/services/ws-service.ts
index 512734ab..5707e9be 100644
--- a/api/services/ws-service.ts
+++ b/api/services/ws-service.ts
@@ -120,7 +120,11 @@ function sendRaw(socket: WebSocket, payload: string): void {
   if (socket.readyState === WebSocket.OPEN) socket.send(payload);
 }
 
-function send(socket: WebSocket, data: ServerToClientMessage): void {
+/**
+ * Sends `data` to one socket unless it is closing. The WebSocket route answers
+ * a client's own message with it: a pong, a vote's ack, an error.
+ */
+export function send(socket: WebSocket, data: ServerToClientMessage): void {
   sendRaw(socket, JSON.stringify(data));
 }
 
diff --git a/api/test/lib/ttl-cache.test.ts b/api/test/lib/ttl-cache.test.ts
new file mode 100644
index 00000000..46904fb8
--- /dev/null
+++ b/api/test/lib/ttl-cache.test.ts
@@ -0,0 +1,37 @@
+import { assertEquals } from "@std/assert";
+import { FakeTime } from "@std/testing/time";
+import { TtlCache } from "../../lib/ttl-cache.ts";
+
+Deno.test("TtlCache: an entry expires ttlMs after it is written", () => {
+  using time = new FakeTime();
+  const cache = new TtlCache<string>(1000, 10);
+  cache.set("a", "x");
+  time.tick(999);
+  assertEquals(cache.get("a"), "x");
+  time.tick(1);
+  assertEquals(cache.get("a"), undefined);
+});
+
+Deno.test("TtlCache: past maxEntries, the oldest write goes first", () => {
+  const cache = new TtlCache<number>(1000, 2);
+  cache.set("a", 1);
+  cache.set("b", 2);
+  // Rewriting "a" makes it the newest, so "b" is the oldest now.
+  cache.set("a", 3);
+  cache.set("c", 4);
+  assertEquals(cache.get("a"), 3);
+  assertEquals(cache.get("b"), undefined);
+  assertEquals(cache.get("c"), 4);
+});
+
+Deno.test("TtlCache: delete with a value spares a newer entry", () => {
+  const cache = new TtlCache<object>(1000, 10);
+  const first = {};
+  const second = {};
+  cache.set("k", first);
+  cache.set("k", second);
+  cache.delete("k", first);
+  assertEquals(cache.get("k"), second);
+  cache.delete("k", second);
+  assertEquals(cache.get("k"), undefined);
+});
diff --git a/api/test/remote/html.test.ts b/api/test/remote/html.test.ts
index 8479a46d..e34dddcc 100644
--- a/api/test/remote/html.test.ts
+++ b/api/test/remote/html.test.ts
@@ -78,6 +78,7 @@ Deno.test("normalizeCssColor", () => {
   assertEquals(normalizeCssColor("#12345"), undefined);
   assertEquals(normalizeCssColor("hsl(0 100% 50%)"), undefined);
   assertEquals(normalizeCssColor("rgb(1,2)"), undefined);
+  assertEquals(normalizeCssColor("transparent"), undefined);
   assertEquals(normalizeCssColor(undefined), undefined);
 });
 
diff --git a/api/test/routes/attachments.test.ts b/api/test/routes/attachments.test.ts
index ed51b8f1..ae1e5db3 100644
--- a/api/test/routes/attachments.test.ts
+++ b/api/test/routes/attachments.test.ts
@@ -10,7 +10,7 @@ import {
   post,
   test,
 } from "../helpers.ts";
-import type { AttachmentResponse } from "../../../shared/dump.ts";
+import type { AttachmentResponse } from "../../../shared/attachment.ts";
 import type { Comment } from "../../../shared/comment.ts";
 import { assertEquals } from "@std/assert";
 import { runStartupTasks } from "../../db/startup.ts";
diff --git a/shared/attachment.ts b/shared/attachment.ts
new file mode 100644
index 00000000..4ea1377c
--- /dev/null
+++ b/shared/attachment.ts
@@ -0,0 +1,37 @@
+/**
+ * Says where an image pasted into a text lives, and how to find it again in
+ * the text. The editor uploads the image and writes a link to it into the
+ * Markdown; the server binds the attachments a saved text links to that text,
+ * and `Markdown` gives a private one the viewer's media token.
+ *
+ * @module
+ */
+
+import { UUID_RE } from "./validation.ts";
+
+/**
+ * Returned by `POST /api/attachments` to the text editor, which inserts the
+ * image into the body as a link to {@link attachmentPath}.
+ */
+export interface AttachmentResponse {
+  id: string;
+}
+
+/** Returns the API path of attachment `id`, to which the API's base is added. */
+export function attachmentPath(id: string): string {
+  return `/api/attachments/${id}`;
+}
+
+const ID = UUID_RE.source.slice(1, -1);
+const ATTACHMENT_LINK_RE = new RegExp(`/api/attachments/(${ID})`, "g");
+const ATTACHMENT_URL_RE = new RegExp(`/api/attachments/${ID}$`);
+
+/** Returns the ids of the attachments `text` links to, in order. */
+export function attachmentIdsIn(text: string): string[] {
+  return [...text.matchAll(ATTACHMENT_LINK_RE)].map((m) => m[1]);
+}
+
+/** Checks whether `url` is an attachment's, with no query yet. */
+export function isAttachmentUrl(url: string): boolean {
+  return ATTACHMENT_URL_RE.test(url);
+}
diff --git a/shared/dump.ts b/shared/dump.ts
index 413188a1..2285ad53 100644
--- a/shared/dump.ts
+++ b/shared/dump.ts
@@ -142,9 +142,15 @@ export interface UpdateDumpRequest {
 }
 
 /**
- * Returned by `POST /api/attachments` to the text editor, which inserts the
- * image into the body as a link to `GET /api/attachments/:id`.
+ * Returns a file dump's version, which the file and thumbnail URLs carry as
+ * `?v=`: the frontend's `dumpFileUrl` and `dumpThumbnailUrl`, and the digest's
+ * thumbnails. It changes whenever the file may have, so the file and thumbnail
+ * routes cache a URL naming the current one for good and revalidate any other.
+ * Without its dates, a dump gets a version no route serves as current.
  */
-export interface AttachmentResponse {
-  id: string;
+export function dumpFileVersion(
+  dump: { fileSize?: number; createdAt?: Date; updatedAt?: Date },
+): string {
+  const at = dump.updatedAt ?? dump.createdAt;
+  return `${dump.fileSize ?? 0}-${at ? at.getTime() : 0}`;
 }
diff --git a/shared/markdown.ts b/shared/markdown.ts
new file mode 100644
index 00000000..8ac21f4e
--- /dev/null
+++ b/shared/markdown.ts
@@ -0,0 +1,28 @@
+/**
+ * Flattens the Markdown users write into one line of plain text, where a
+ * quote of it can't render markup: the chat's reply previews and the digest
+ * email's quotes of a comment or a note.
+ *
+ * @module
+ */
+
+/**
+ * Returns the Markdown `md` as one line of plain text. Images become their alt
+ * text, or `imageLabel` without one, and links their text. It also takes a
+ * body cut short, possibly inside a link, as the server cuts a chat message's
+ * `replyToBody`: the unfinished link then shows as its text, without half a
+ * URL.
+ */
+export function markdownToPlainText(md: string, imageLabel: string): string {
+  return md
+    // An image or link cut short at the end loses its unfinished target.
+    .replace(/!\[([^\]]*)\]\([^)]*$/, (_, alt: string) => alt || imageLabel)
+    .replace(/(^|[^!])\[([^\]]*)\]\([^)]*$/, "$1$2")
+    .replace(/!\[[^\]]*$/, "")
+    .replace(/!\[([^\]]*)\]\([^)]*\)/g, (_, alt: string) => alt || imageLabel)
+    .replace(/\[([^\]]*)\]\([^)]*\)/g, "$1")
+    .replace(/^\s{0,3}(#{1,6}|>|[-*+]|\d+\.)\s+/gm, "")
+    .replace(/```\w*|\*\*|__|~~|[*`]/g, "")
+    .replace(/\s+/g, " ")
+    .trim();
+}
diff --git a/shared/test/markdown.test.ts b/shared/test/markdown.test.ts
new file mode 100644
index 00000000..958c2f90
--- /dev/null
+++ b/shared/test/markdown.test.ts
@@ -0,0 +1,35 @@
+import { assertEquals } from "@std/assert";
+import { markdownToPlainText } from "../markdown.ts";
+
+Deno.test("markdownToPlainText: links become their text, images their alt or a label", () => {
+  assertEquals(
+    markdownToPlainText("see [this](https://x.y) now", "img"),
+    "see this now",
+  );
+  assertEquals(markdownToPlainText("![cat](a.png)", "Image"), "cat");
+  assertEquals(markdownToPlainText("![](a.png)", "Image"), "Image");
+});
+Deno.test("markdownToPlainText: markup characters and block markers go", () => {
+  assertEquals(
+    markdownToPlainText("# Title\n> quote\n- **bold** `code`", ""),
+    "Title quote bold code",
+  );
+});
+Deno.test("markdownToPlainText: a body cut off mid-link doesn't show a half URL", () => {
+  assertEquals(
+    markdownToPlainText("look [here](https://exa", "img"),
+    "look here",
+  );
+  assertEquals(
+    markdownToPlainText("pic ![alt](https://exa", "Image"),
+    "pic alt",
+  );
+  assertEquals(
+    markdownToPlainText("pic ![](https://exa", "Image"),
+    "pic Image",
+  );
+  assertEquals(markdownToPlainText("pic ![unfinished", "Image"), "pic");
+});
+Deno.test("markdownToPlainText: whitespace collapses to one line", () => {
+  assertEquals(markdownToPlainText("a\n\n  b\tc", ""), "a b c");
+});
diff --git a/shared/url.ts b/shared/url.ts
new file mode 100644
index 00000000..c8eb1db4
--- /dev/null
+++ b/shared/url.ts
@@ -0,0 +1,28 @@
+/**
+ * Names the site a link points at, as the app shows it: a link dump's title
+ * when its page has none, a preview's site name, the digest's domain line and
+ * the tint of a card without artwork. The server's link checks key on it too
+ * (`canonical-url.ts`), so a link reads as the same site everywhere.
+ *
+ * @module
+ */
+
+/**
+ * Returns `url`'s host without a leading `www.`; `URL` has already lowered
+ * its case. `www.youtube.com` and `youtube.com` give `youtube.com`.
+ */
+export function siteHostOf(url: URL): string {
+  return url.hostname.replace(/^www\./, "");
+}
+
+/**
+ * Returns {@link siteHostOf} the URL `raw`, or undefined when `raw` isn't
+ * one.
+ */
+export function siteHost(raw: string): string | undefined {
+  try {
+    return siteHostOf(new URL(raw));
+  } catch {
+    return undefined;
+  }
+}
diff --git a/shared/validation.ts b/shared/validation.ts
index a4057db3..8ad5aac3 100644
--- a/shared/validation.ts
+++ b/shared/validation.ts
@@ -46,6 +46,14 @@ export const RENAME_LIMITS = {
   SLUG: { count: 3, windowMs: 24 * 3_600_000 },
 } as const;
 
+/**
+ * Matches an id the server makes with `crypto.randomUUID()`. A route checks
+ * one before it names a file on disk, and shared/attachment.ts finds the
+ * attachments a text links by it.
+ */
+export const UUID_RE: RegExp =
+  /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/;
+
 /**
  * Matches an email address well enough to catch a typo; only the reset email
  * arriving proves it.
diff --git a/src/components/editor/Markdown.tsx b/src/components/editor/Markdown.tsx
index 30f1edda..3f06fbd7 100644
--- a/src/components/editor/Markdown.tsx
+++ b/src/components/editor/Markdown.tsx
@@ -12,6 +12,7 @@ import remarkGfm from "remark-gfm";
 import { useAuth } from "../../hooks/auth/useAuth.ts";
 import { userPath } from "../../../shared/paths.ts";
 import { EVERYONE } from "../../../shared/mention.ts";
+import { isAttachmentUrl } from "../../../shared/attachment.ts";
 
 /** Passed to {@link Markdown} by any component that shows a user's text. */
 export interface MarkdownProps {
@@ -154,8 +155,6 @@ function internalPath(href: string | undefined): string | null {
 
 type Components = React.ComponentProps<typeof ReactMarkdown>["components"];
 
-const ATTACHMENT_PATH_RE = /\/api\/attachments\/[0-9a-f-]{36}$/;
-
 /**
  * Renders an image of the text. An attachment gets the viewer's media token,
  * which the server asks for when the text linking it is private; any other
@@ -166,7 +165,7 @@ function MarkdownImage(
 ): React.JSX.Element {
   const { mediaToken } = useAuth();
   let url = typeof src === "string" ? src : undefined;
-  if (url && mediaToken && ATTACHMENT_PATH_RE.test(url)) {
+  if (url && mediaToken && isAttachmentUrl(url)) {
     url = `${url}?token=${encodeURIComponent(mediaToken)}`;
   }
   return <img src={url} alt={alt ?? ""} loading="lazy" decoding="async" />;
diff --git a/src/components/editor/TextEditor.tsx b/src/components/editor/TextEditor.tsx
index 4ae474fa..1e7f628b 100644
--- a/src/components/editor/TextEditor.tsx
+++ b/src/components/editor/TextEditor.tsx
@@ -27,6 +27,7 @@ import { useEmojiTrigger } from "../../hooks/editor/useEmojiTrigger.ts";
 import { useAnchoredFloat } from "../../hooks/shared/useAnchoredFloat.ts";
 import { useAuth } from "../../hooks/auth/useAuth.ts";
 import { API_URL } from "../../config/api.ts";
+import { attachmentPath } from "../../../shared/attachment.ts";
 
 /**
  * Exposed through the ref of {@link TextEditor}, so that the component that
@@ -342,7 +343,7 @@ export const TextEditor: React.ForwardRefExoticComponent<
           });
           const json = await res.json();
           if (!json.success) throw new Error(json.error?.message);
-          images.push(`![](${API_URL}/api/attachments/${json.data.id})`);
+          images.push(`![](${API_URL}${attachmentPath(json.data.id)})`);
         } catch {
           setNotice(
             files.length > 1
diff --git a/src/test/components/dump/FilePreview.test.tsx b/src/test/components/dump/FilePreview.test.tsx
index 9b3fcbbe..1d215e0b 100644
--- a/src/test/components/dump/FilePreview.test.tsx
+++ b/src/test/components/dump/FilePreview.test.tsx
@@ -4,11 +4,8 @@ import { userEvent } from "@testing-library/user-event";
 import { FilePreview } from "../../../components/dump/FilePreview.tsx";
 import { makeDump, makeSession } from "../../fixtures.ts";
 import { renderWithContexts } from "../../render.tsx";
-import {
-  dumpFileUrl,
-  dumpFileVersion,
-  dumpThumbnailUrl,
-} from "../../../utils/shared/urls.ts";
+import { dumpFileUrl, dumpThumbnailUrl } from "../../../utils/shared/urls.ts";
+import { dumpFileVersion } from "../../../../shared/dump.ts";
 
 describe("FilePreview", () => {
   const session = makeSession();
diff --git a/src/test/utils/chat/text.test.ts b/src/test/utils/chat/text.test.ts
index ff99e60c..fe063dd5 100644
--- a/src/test/utils/chat/text.test.ts
+++ b/src/test/utils/chat/text.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, test } from "vitest";
-import { markdownToPlainText, mentionsUser } from "../../../utils/chat/text.ts";
+import { mentionsUser } from "../../../utils/chat/text.ts";
 
 describe("mentionsUser", () => {
   test("case-insensitive whole-word match", () => {
@@ -15,33 +15,3 @@ describe("mentionsUser", () => {
     expect(mentionsUser("@bob @carol @alice", "alice")).toBe(true);
   });
 });
-
-describe("markdownToPlainText", () => {
-  test("links become their text, images their alt or a label", () => {
-    expect(markdownToPlainText("see [this](https://x.y) now", "img")).toBe(
-      "see this now",
-    );
-    expect(markdownToPlainText("![cat](a.png)", "Image")).toBe("cat");
-    expect(markdownToPlainText("![](a.png)", "Image")).toBe("Image");
-  });
-  test("markup characters and block markers go", () => {
-    expect(markdownToPlainText("# Title\n> quote\n- **bold** `code`", "")).toBe(
-      "Title quote bold code",
-    );
-  });
-  test("a body cut off mid-link doesn't show a half URL", () => {
-    expect(markdownToPlainText("look [here](https://exa", "img")).toBe(
-      "look here",
-    );
-    expect(markdownToPlainText("pic ![alt](https://exa", "Image")).toBe(
-      "pic alt",
-    );
-    expect(markdownToPlainText("pic ![](https://exa", "Image")).toBe(
-      "pic Image",
-    );
-    expect(markdownToPlainText("pic ![unfinished", "Image")).toBe("pic");
-  });
-  test("whitespace collapses to one line", () => {
-    expect(markdownToPlainText("a\n\n  b\tc", "")).toBe("a b c");
-  });
-});
diff --git a/src/utils/chat/message.ts b/src/utils/chat/message.ts
index 1d4639e6..acbb67f7 100644
--- a/src/utils/chat/message.ts
+++ b/src/utils/chat/message.ts
@@ -6,7 +6,7 @@
  */
 
 import { t } from "@lingui/core/macro";
-import { markdownToPlainText } from "./text.ts";
+import { markdownToPlainText } from "../../../shared/markdown.ts";
 import { isEmojiOnly } from "../editor/text.ts";
 import { VALIDATION } from "../../../shared/validation.ts";
 
diff --git a/src/utils/chat/text.ts b/src/utils/chat/text.ts
index 6c9f7023..97f639ce 100644
--- a/src/utils/chat/text.ts
+++ b/src/utils/chat/text.ts
@@ -1,6 +1,5 @@
 /**
- * Finds the mentions in a chat message, and flattens its Markdown into reply
- * previews.
+ * Finds the mentions of the signed-in user in a chat message.
  *
  * @module
  */
@@ -15,24 +14,3 @@ import { mentionedUsernames } from "../../../shared/mention.ts";
 export function mentionsUser(body: string, username: string): boolean {
   return mentionedUsernames(body).includes(username.toLowerCase());
 }
-
-/**
- * Returns the Markdown `md` as one line of plain text. Images become their alt
- * text, or `imageLabel` without one, and links their text. It takes a
- * `replyToBody`, which the server cuts short, possibly inside a link: the
- * unfinished link then shows as its text, without half a URL. `snippet` calls
- * it.
- */
-export function markdownToPlainText(md: string, imageLabel: string): string {
-  return md
-    // An image or link cut short at the end loses its unfinished target.
-    .replace(/!\[([^\]]*)\]\([^)]*$/, (_, alt: string) => alt || imageLabel)
-    .replace(/(^|[^!])\[([^\]]*)\]\([^)]*$/, "$1$2")
-    .replace(/!\[[^\]]*$/, "")
-    .replace(/!\[([^\]]*)\]\([^)]*\)/g, (_, alt: string) => alt || imageLabel)
-    .replace(/\[([^\]]*)\]\([^)]*\)/g, "$1")
-    .replace(/^\s{0,3}(#{1,6}|>|[-*+]|\d+\.)\s+/gm, "")
-    .replace(/```\w*|\*\*|__|~~|[*`]/g, "")
-    .replace(/\s+/g, " ")
-    .trim();
-}
diff --git a/src/utils/shared/thumbnailTint.ts b/src/utils/shared/thumbnailTint.ts
index 242198b8..bfc7de17 100644
--- a/src/utils/shared/thumbnailTint.ts
+++ b/src/utils/shared/thumbnailTint.ts
@@ -6,6 +6,8 @@
  * @module
  */
 
+import { siteHost } from "../../../shared/url.ts";
+
 /**
  * Checks whether `value` is a lowercase `#rrggbb` colour. An `accentColor`
  * comes from a dumped page's `theme-color`, which anyone controls, and a custom
@@ -37,15 +39,6 @@ function hueFromString(seed: string): string {
   return `hsl(${hashString(seed) % 360} 55% 55%)`;
 }
 
-function hostnameOf(url: string | undefined): string | undefined {
-  if (!url) return undefined;
-  try {
-    return new URL(url).hostname.replace(/^www\./, "");
-  } catch {
-    return undefined;
-  }
-}
-
 /**
  * Returns the page's own colour when it declared a valid one, or else a hue
  * drawn from `seed` or the hostname of `url`, so that every card gets a tint,
@@ -59,7 +52,7 @@ export function tintFor(
   },
 ): string {
   if (isSafeHex(accentColor)) return accentColor;
-  return hueFromString(seed ?? hostnameOf(url) ?? url ?? "");
+  return hueFromString(seed ?? (url && siteHost(url)) ?? url ?? "");
 }
 
 /**
@@ -70,7 +63,7 @@ export function initialsFor(
   siteName: string | undefined,
   url: string | undefined,
 ): string {
-  const source = siteName?.trim() || hostnameOf(url) || url || "";
+  const source = siteName?.trim() || (url && siteHost(url)) || url || "";
   const letter = [...source].find((ch) => /\p{L}|\p{N}/u.test(ch));
   return (letter ?? "?").toUpperCase();
 }
diff --git a/src/utils/shared/urls.ts b/src/utils/shared/urls.ts
index 2e275d74..70db8c72 100644
--- a/src/utils/shared/urls.ts
+++ b/src/utils/shared/urls.ts
@@ -13,6 +13,7 @@ import {
   type OwnedItem,
   playlistPath,
 } from "../../../shared/paths.ts";
+import { dumpFileVersion } from "../../../shared/dump.ts";
 
 /**
  * Passed to {@link dumpFileUrl} and {@link dumpThumbnailUrl}, usually as a
@@ -26,18 +27,6 @@ export type VersionedDump = {
   updatedAt?: Date;
 };
 
-/**
- * Returns a file dump's version, as `dumpFileVersion` in
- * api/services/dump-service.ts computes it, so the two must change together.
- * It changes whenever the file can have. The server lets the browser cache a
- * URL that carries the current version for good, and has it revalidate any
- * other.
- */
-export function dumpFileVersion(dump: Omit<VersionedDump, "id">): string {
-  const at = dump.updatedAt ?? dump.createdAt;
-  return `${dump.fileSize ?? 0}-${at ? at.getTime() : 0}`;
-}
-
 /**
  * Returns the URL of the uploaded file of `dump`. The caller passes the
  * session's media token (`useAuth`'s `mediaToken`) as `token`, which goes in