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>

frontend/src/components/subscriptions/SourceFormDialog.vue

@@ -52,6 +52,9 @@
               placeholder="Todding" hint="Comma-separated. Empty takes everyone's."
               persistent-hint class="mt-2"
             />
+            <div v-if="platform === 'discord' && authorNames.length" class="text-caption mt-2">
+              {{ authorNames.join(', ') }}
+            </div>
             <p class="text-caption mt-2" style="opacity: 0.75">
               More per-platform fields land here over time. Use Advanced JSON for everything else.
             </p>
@@ -112,6 +115,15 @@ const structuredAuthors = ref('')
 const otherConfig = ref({})
 const STRUCTURED_KEYS = ['videos', 'since', 'discord_authors']
 
+// The extension's poster picker stores ids (#4488) and the name each was
+// picked under, so a list of numbers still says who it means.
+const authorNames = computed(() => {
+  const labels = otherConfig.value.discord_author_labels || {}
+  return splitAuthors(structuredAuthors.value)
+    .map(a => (labels[a] ? `${labels[a]} (${a})` : null))
+    .filter(Boolean)
+})
+
 function splitAuthors(txt) {
   return (txt || '').split(',').map(s => s.trim()).filter(Boolean)
 }