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/database.test.ts

@@ -103,8 +103,8 @@ describe('Database Client', () => {
       const tables = db
         .prepare("SELECT name FROM sqlite_master WHERE type='table' AND name NOT LIKE 'sqlite_%'")
         .all() as { name: string }[];
-      // processed_posts, sent_messages, successful_messages, activity_log
-      expect(tables.length).toBe(4);
+      // processed_posts, sent_messages, successful_messages, activity_log, reddit_users
+      expect(tables.length).toBe(5);
     });
 
     it('processed_posts has correct schema', () => {