feat: the extension adds Discord channels to an artist you pick, and its tests gate the XPI (milestone 429)

Server (#4420)
- extension_service gains a Discord pattern (server or channel, jump links,
  ptb/canary; not DMs or threads), mirrored in platforms.js and pinned by
  the shared artist-url-samples.json.
- probe on a Discord URL matches the source by ids under any artist, reports
  a whole-server source as covering the channel, suggests the artist who owns
  another source on the same server, and names server/channel via the stored
  token (best-effort, bounded, no rate-limit waits).
- quick-add takes artist_id / artist_name; Discord URLs are stored canonical.

Extension (#4421, #4422)
- Content script on discord.com; SPA navigation by URL polling (the old
  pushState patch ran in the isolated world and never fired); stale probes
  are dropped.
- Discord chip opens an Add panel: this channel or the whole server, and the
  suggested artist / a search / a new name.
- Popup: sources show artist, platform and state; a Discord token export is
  verified by FC and the result shown. Token capture covers ptb/canary.
- Pure logic in lib/chip.js and lib/popup-format.js, with specs.

CI (#4423)
- extension.yml's lane (web-ext lint, vitest, XPI contents) moves into
  build.yml as extension-test and joins the needs of sign-extension,
  build-web and build-agent. As a separate workflow it gated nothing: a red
  extension suite still signed and shipped the XPI (rule 177).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

ci-requirements.md

@@ -15,7 +15,7 @@ git.fabledsword.com/bvandeusen/ci-python:3.14
 
 ## Secondary runtime image
 
-node:24-bookworm-slim — `.forgejo/workflows/extension.yml` only.
+node:24-bookworm-slim — `build.yml`'s `extension-test` job only.
 
 `.forgejo/workflows/release.yml` runs on `ci-python:3.14` like everything else
 and installs nothing: it needs git and stdlib python, and builds no image.
@@ -29,8 +29,8 @@ per `docs/process.md`'s "add deps to the image when used by >1 project".
 
 - `pip install -r requirements.txt pytest pytest-asyncio` — in `backend-lint-and-test` and `integration` jobs
 - `npm install --no-audit --no-fund` — in `frontend-build` job
-- `npm install --no-audit --no-fund` — in `extension.yml`'s `lint` job (web-ext + vitest)
-- `unzip` — in `extension.yml`'s "Verify XPI contents" step, installed via apt
+- `npm install --no-audit --no-fund` — in `build.yml`'s `extension-test` job (web-ext + vitest)
+- `unzip` — in `extension-test`'s "Verify XPI contents" step, installed via apt
   only when absent (`node:24-bookworm-slim` may or may not carry it). Debian
   package, ~2s. Not worth baking into a shared image for a single consumer, per
   `docs/process.md`'s ">1 project" rule.