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/background/background.js

@@ -276,11 +276,26 @@ browser.runtime.onMessage.addListener(async (msg) => {
           artistId: msg.artistId ?? null,
           artistName: msg.artistName ?? null,
           usePlatformName: msg.usePlatformName === true,
+          discordAuthors: Array.isArray(msg.discordAuthors) ? msg.discordAuthors : null,
         });
       } catch (e) {
         return { error: e.message };
       }
 
+    case 'DISCORD_POSTERS':
+      try {
+        return await api.getDiscordPosters(msg.url);
+      } catch (e) {
+        return { error: e.message };
+      }
+
+    case 'SET_DISCORD_POSTERS':
+      try {
+        return await api.setDiscordPosters(msg.url, msg.authors || []);
+      } catch (e) {
+        return { error: e.message };
+      }
+
     case 'SEARCH_ARTISTS':
       try {
         return { artists: await api.searchArtists(msg.q || '') };