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

backend/app/services/discord_client.py

@@ -510,6 +510,24 @@ class DiscordClient:
 
     # -- verify ------------------------------------------------------------
 
+    def describe(self, server_id: str | None, channel_id: str | None) -> dict:
+        """The display names behind a server/channel pair, for the browser
+        extension's Add panel. Best-effort per name: one that can't be read
+        comes back None, and the other is still returned."""
+        out: dict = {"server": None, "channel": None, "parent": None}
+        if server_id:
+            try:
+                out["server"] = (self._get(f"/guilds/{server_id}") or {}).get("name") or None
+            except DiscordAPIError:
+                pass
+        if channel_id:
+            try:
+                meta = self._parse_channel(self._get(f"/channels/{channel_id}"))
+                out["channel"] = meta.get("channel") or None
+            except (DiscordAPIError, AttributeError):
+                pass
+        return out
+
     def verify_auth(self, url: str) -> tuple[bool | None, str]:
         """Is the token valid, and can it see what the source names?"""
         try: