Stop re-attempting /joined_members on rooms we can't read

Reddit's homeserver returns 404 from /_matrix/client/v3/rooms/{id}/joined_members
for rooms we've left, been kicked from, or never fully joined — which is
roughly every chat that wasn't accepted by the recipient. The previous
enrichUsernames pass retried these on every delta sync, flooding the
dashboard log with debug-level 404s. Track failures in an in-memory
unresolvable set so each room is tried at most once per process.

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

src/services/chat/service.ts

@@ -104,6 +104,8 @@ export class ChatService {
   private cache: ChatCache;
   private queue: Promise<unknown> = Promise.resolve();
   private readonly t2ToUsername = new Map<string, string>();
+  /** Rooms whose /joined_members lookup has already failed — don't retry. */
+  private readonly unresolvableRooms = new Set<string>();
   private syncingKind: 'full' | 'delta' | null = null;
   private progress: SyncProgress | null = null;
   private progressListener: SyncProgressListener | null = null;
@@ -1006,6 +1008,7 @@ export class ChatService {
     const roomT2: Map<string, string> = new Map(); // roomId -> bare t2 id
     for (const chat of this.cache.get()) {
       if (!chat.roomId) continue;
+      if (this.unresolvableRooms.has(chat.roomId)) continue;
       const needsLookup =
         !chat.username ||
         chat.username === 'unknown' ||
@@ -1023,8 +1026,16 @@ export class ChatService {
           if (!chat.roomName || chat.roomName === 'unknown' || chat.roomName.startsWith('@')) {
             chat.roomName = chat.username;
           }
+        } else {
+          // No t2-shaped member in the room (probably empty or only us
+          // after the other party left). Don't keep retrying every sync.
+          this.unresolvableRooms.add(chat.roomId);
         }
       } catch (err) {
+        // 404/403 means we can't read membership for this room (left, kicked,
+        // or never fully joined). Cache the failure so the next delta doesn't
+        // spam the homeserver — and the log — with the same lookup.
+        this.unresolvableRooms.add(chat.roomId);
         this.logger.debug(`[chat] joined_members lookup failed for ${chat.roomId}:`, err);
       }
     }