v4: what the server and the client both state is declared once — several rules were written out in two or more files, some already drifting apart. A link's site name (its host without www.) was computed in five places, now shared/url.ts's siteHost; dumpFileVersion, which the frontend and the server had to keep equal by hand, is one function in shared/dump.ts; the Markdown flattening of the chat's reply previews and the digest's quotes is shared/markdown.ts's markdownToPlainText, so the digest also keeps an image's alt text; an attachment's URL and how to find one in a text are shared/attachment.ts, which the editor, Markdown and attachment-service read, its id pattern the new UUID_RE that the file and thumbnail routes check ids with. On the server, the Bandcamp, YouTube playlist and link preview caches use one TtlCache (api/lib/ttl-cache.ts) instead of three copies of the same expiry and eviction; the link previews' theme-color and the emails' theme tokens read colours with one parser (api/lib/css-color.ts), and a transparent theme-color now gives no tint; the WebSocket route sends through ws-service's send, the welcome included, so it is type-checked; fetch.ts strips an IPv6 literal's brackets with public-address's bareHostname; and a category's slug is checked by the same validateSlug as a dump's, with its own length. The migrations keep their frozen copies, as they document. 272 backend tests and 677 frontend tests pass, and deno check, tsc, eslint and doc:lint are clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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",