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>

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}`;
 }