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/database/client.ts

@@ -66,6 +66,15 @@ export function runMigrations(db: Database.Database): void {
       image_url TEXT
     );
 
+    CREATE TABLE IF NOT EXISTS reddit_users (
+      t2_id TEXT PRIMARY KEY,
+      username TEXT NOT NULL,
+      mxid TEXT,
+      updated_at INTEGER NOT NULL
+    );
+    CREATE INDEX IF NOT EXISTS idx_reddit_users_username
+      ON reddit_users(username);
+
     CREATE INDEX IF NOT EXISTS idx_posts_subreddit
       ON processed_posts(subreddit);
     CREATE INDEX IF NOT EXISTS idx_posts_processed_at