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>

tests/chat-service-username-persist.test.ts

@@ -0,0 +1,92 @@
+import { describe, it, expect } from 'vitest';
+import os from 'os';
+import path from 'path';
+import Database from 'better-sqlite3';
+import { runMigrations } from '../src/database/client.js';
+import { RedditUsersRepository } from '../src/database/repositories.js';
+import { ChatService } from '../src/services/chat/service.js';
+import { ChatCache } from '../src/services/chat/cache.js';
+import { MatrixClient } from '../src/services/chat/matrix/client.js';
+import { createMockLogger } from './mockLogger.js';
+
+const creds = {
+  homeserver: 'https://matrix.example.test',
+  accessToken: 'tok',
+  userId: '@me:example.test',
+  capturedAt: Date.now(),
+};
+
+function buildMatrix() {
+  const logger = createMockLogger();
+  const fetchImpl = (() =>
+    Promise.resolve(
+      new Response('{}', { status: 200, headers: { 'content-type': 'application/json' } }),
+    )) as typeof fetch;
+  return new MatrixClient(creds, logger, { minRequestIntervalMs: 0, fetchImpl });
+}
+
+describe('ChatService username persistence', () => {
+  it('seeds the in-memory cache from RedditUsersRepository on construction', () => {
+    const db = new Database(':memory:');
+    runMigrations(db);
+    const repo = new RedditUsersRepository(db);
+    repo.upsert({ t2Id: 't2_abc', username: 'alice' });
+
+    const logger = createMockLogger();
+    const svc = new ChatService(
+      buildMatrix(),
+      logger,
+      undefined, undefined, undefined, undefined, undefined, undefined,
+      repo,
+    );
+
+    expect(svc.resolveSenderDisplay('@t2_abc:reddit.com')).toBe('alice');
+    expect(svc.resolveSenderDisplay('t2_abc')).toBe('alice');
+    db.close();
+  });
+
+  it('falls back to t2 form when no mapping is known', () => {
+    const logger = createMockLogger();
+    const svc = new ChatService(buildMatrix(), logger);
+    expect(svc.resolveSenderDisplay('@t2_unknown:reddit.com')).toBe('t2_unknown');
+  });
+
+  it('upgrades cached chats with t2_ usernames from the db map on loadCache', async () => {
+    const db = new Database(':memory:');
+    runMigrations(db);
+    const repo = new RedditUsersRepository(db);
+    repo.upsert({ t2Id: 't2_abc', username: 'alice' });
+
+    const logger = createMockLogger();
+    const tmpFile = path.join(os.tmpdir(), `chat-cache-${Date.now()}-${Math.random()}.json`);
+    const cache = new ChatCache(logger, tmpFile);
+    cache.set([
+      {
+        username: 't2_abc',
+        roomName: 't2_abc',
+        lastMessageTime: Date.now(),
+        lastMessageSnippet: 'hi',
+        isFromUs: false,
+        unread: false,
+        url: 'https://chat.reddit.com/room/x',
+        roomId: '!room:reddit.com',
+        messages: [],
+      },
+    ]);
+
+    const svc = new ChatService(
+      buildMatrix(),
+      logger,
+      undefined, undefined, undefined, undefined,
+      cache,
+      undefined,
+      repo,
+    );
+
+    await svc.loadCache();
+    const chats = await svc.getCachedConversations();
+    expect(chats[0].username).toBe('alice');
+    expect(chats[0].roomName).toBe('alice');
+    db.close();
+  });
+});