Fall back to /joined_members when delta sync omits the other party

Reddit's homeserver sometimes returns a room in /sync with no
m.heroes and no joined m.room.member state events, leaving us with
no t2 ID to resolve and the chat stuck on 'unknown'. enrichUsernames
now hits /_matrix/client/v3/rooms/{roomId}/joined_members for any
chat whose username is 'unknown' or a bare MXID, stamps the t2 ID,
and lets the existing resolver fill in the real Reddit username.

Also treats a cached 'unknown' username as stale in roomToActiveChat
so subsequent deltas re-attempt the lookup instead of pinning it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/services/chat/service.ts

@@ -838,7 +838,12 @@ export class ChatService {
       .filter((s): s is string => !!s);
 
     let username = prev?.username ?? null;
-    if (!username || username.startsWith('t2_') || username.startsWith('@')) {
+    if (
+      !username ||
+      username === 'unknown' ||
+      username.startsWith('t2_') ||
+      username.startsWith('@')
+    ) {
       const resolved = otherT2
         .map((id) => opts.knownUsernames.get(id.replace(/^t2_/, '')))
         .find((x): x is string => !!x);
@@ -994,12 +999,45 @@ export class ChatService {
    */
   async enrichUsernames(): Promise<void> {
     if (!this.usernameResolver) return;
+    // First, for any chats whose username is still 'unknown' (heroes/state in
+    // the delta didn't surface the other member), fetch the room's joined
+    // members and stamp the t2 ID onto the chat so the resolver below can
+    // pick it up.
+    const roomT2: Map<string, string> = new Map(); // roomId -> bare t2 id
+    for (const chat of this.cache.get()) {
+      if (!chat.roomId) continue;
+      const needsLookup =
+        !chat.username ||
+        chat.username === 'unknown' ||
+        chat.username.startsWith('@');
+      if (!needsLookup) continue;
+      try {
+        const members = await this.withAuth(() => this.matrix.getJoinedMembers(chat.roomId!));
+        const otherMxid = Object.keys(members).find((m) => m !== this.matrix.userId);
+        const t2 = otherMxid?.match(/^@(t2_[^:]+):/)?.[1];
+        if (t2) {
+          const bare = t2.replace(/^t2_/, '');
+          roomT2.set(chat.roomId, bare);
+          const known = this.t2ToUsername.get(bare);
+          chat.username = known ?? t2;
+          if (!chat.roomName || chat.roomName === 'unknown' || chat.roomName.startsWith('@')) {
+            chat.roomName = chat.username;
+          }
+        }
+      } catch (err) {
+        this.logger.debug(`[chat] joined_members lookup failed for ${chat.roomId}:`, err);
+      }
+    }
+
     const needed: string[] = [];
     for (const chat of this.cache.get()) {
       const t2 = this.extractT2(chat);
       if (t2 && !this.t2ToUsername.has(t2)) needed.push(`t2_${t2}`);
     }
-    if (needed.length === 0) return;
+    if (needed.length === 0) {
+      if (roomT2.size > 0) await this.cache.save();
+      return;
+    }
     try {
       const got = await this.usernameResolver.resolveT2Ids(needed);
       let updated = 0;
@@ -1015,7 +1053,7 @@ export class ChatService {
           }
         }
       }
-      if (updated > 0) {
+      if (updated > 0 || roomT2.size > 0) {
         this.logger.info(`[chat] enriched ${updated} chats with usernames`);
         await this.cache.save();
       }