feat: the extension picks a Discord source's posters from who posts in the channel, stored by id (#4488)

On a Discord channel page the Add panel lists who posted in the newest 200
messages, the creator the server is named for (or owned by) ticked. On a
channel FabledCurator already follows, the chip opens the same list with Save
and Open artist. The list is kept as user ids, so a rename never quietly stops
it matching; the name each was picked under rides beside it for display, and
the Subscriptions dialog shows it.

- DiscordClient.recent_posters + rank_posters (owner / name-matches-server;
  bots never suggested; it suggests, the operator ticks).
- GET/POST /api/extension/discord/posters; quick-add takes discord_authors.

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

extension/content/content-script.css

@@ -100,3 +100,7 @@
 /* The panel's own [hidden] — its rows are display:flex, which beats the UA's. */
 .fc-panel [hidden] { display: none !important; }
 .fc-panel__rename { margin-top: 8px; font-size: 13px; }
+/* Poster picker (#4488): who posts in the channel, the likely creator ticked. */
+.fc-panel__posters { max-height: 220px; overflow-y: auto; }
+.fc-panel__poster { flex-wrap: wrap; }
+.fc-panel__poster-meta { flex-basis: 100%; padding-left: 21px; font-size: 11px; color: rgb(170, 166, 156); }