Persist t2→username mappings in reddit_users table

Adds a durable mapping so the UI never has to show raw t2_xxx IDs or
MXIDs just because the in-memory cache is cold after a restart.
ChatService seeds from the table on construction and writes through
every resolution path; cached chats whose username is still t2_xxx
get upgraded from the DB on loadCache.

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

src/bootstrap.ts

@@ -7,6 +7,7 @@ import {
   MessagesRepository,
   SuccessfulMessagesRepository,
   ActivityLogRepository,
+  RedditUsersRepository,
 } from './database/repositories.js';
 import { BrowserClient } from './browser/client.js';
 import { RedditMessagePage } from './browser/message-page.js';
@@ -79,6 +80,7 @@ export async function bootstrap(config: Config, logger: Logger): Promise<AppCont
   const messagesRepo = new MessagesRepository(db);
   const successfulMessagesRepo = new SuccessfulMessagesRepository(db);
   const activityLogRepo = new ActivityLogRepository(db);
+  const redditUsersRepo = new RedditUsersRepository(db);
   logger.info('Database initialized');
 
   // Browser
@@ -208,22 +210,23 @@ export async function bootstrap(config: Config, logger: Logger): Promise<AppCont
     usernameResolver,
     undefined,
     messagesRepo,
+    redditUsersRepo,
   );
 
   const mediaSyncService = new MediaSyncService(chatService, logger, config.media.syncIntervalMs);
   const rateLimiter = new RateLimiter(config, logger);
 
   await chatService.loadCache();
-  // Lazy enrichment in the background — first sync may show t2 IDs briefly.
-  chatService.enrichUsernames().catch((err) => logger.debug('enrichUsernames failed:', err));
 
   // Fire-and-forget startup sync: full if we've never backfilled, delta
   // otherwise. Non-blocking so the rest of the bot can come up immediately;
   // the dashboard's sync-status banner shows progress.
   chatService
     .runStartupSync()
-    .then(({ kind, chats }) => {
+    .then(async ({ kind, chats }) => {
       logger.info(`[chat] startup sync (${kind}) complete: ${chats} chats`);
+      // Resolve t2 IDs now that the full chat list is available.
+      await chatService.enrichUsernames();
     })
     .catch((err) => logger.error('[chat] startup sync failed:', err));