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

src/database/repositories.ts

@@ -566,3 +566,83 @@ export class SuccessfulMessagesRepository {
     };
   }
 }
+
+export interface RedditUserRecord {
+  t2Id: string;
+  username: string;
+  mxid: string | null;
+  updatedAt: number;
+}
+
+/**
+ * Durable mapping of Reddit account IDs (`t2_xxx`) to current usernames.
+ * Populated as the chat sync resolves IDs via Reddit's alfred/user_data
+ * endpoints; read back on startup so we never have to show raw `t2_xxx`
+ * or MXIDs in the UI just because the in-memory cache is cold.
+ */
+export class RedditUsersRepository {
+  private upsertStmt: Database.Statement;
+  private getByT2Stmt: Database.Statement;
+  private getByUsernameStmt: Database.Statement;
+  private getAllStmt: Database.Statement;
+
+  constructor(private db: Database.Database) {
+    this.upsertStmt = db.prepare(`
+      INSERT INTO reddit_users (t2_id, username, mxid, updated_at)
+      VALUES (@t2Id, @username, @mxid, @updatedAt)
+      ON CONFLICT(t2_id) DO UPDATE SET
+        username   = excluded.username,
+        mxid       = COALESCE(excluded.mxid, reddit_users.mxid),
+        updated_at = excluded.updated_at
+      WHERE excluded.username != ''
+    `);
+    this.getByT2Stmt = db.prepare(`SELECT * FROM reddit_users WHERE t2_id = ?`);
+    this.getByUsernameStmt = db.prepare(`SELECT * FROM reddit_users WHERE username = ? COLLATE NOCASE`);
+    this.getAllStmt = db.prepare(`SELECT * FROM reddit_users`);
+  }
+
+  upsert(record: { t2Id: string; username: string; mxid?: string | null }): void {
+    if (!record.t2Id || !record.username) return;
+    if (record.username.startsWith('t2_') || record.username.startsWith('@')) return;
+    const t2Id = record.t2Id.startsWith('t2_') ? record.t2Id : `t2_${record.t2Id}`;
+    this.upsertStmt.run({
+      t2Id,
+      username: record.username,
+      mxid: record.mxid ?? null,
+      updatedAt: Date.now(),
+    });
+  }
+
+  upsertMany(records: Array<{ t2Id: string; username: string; mxid?: string | null }>): void {
+    const tx = this.db.transaction((items: typeof records) => {
+      for (const r of items) this.upsert(r);
+    });
+    tx(records);
+  }
+
+  getByT2(t2Id: string): RedditUserRecord | null {
+    const normalized = t2Id.startsWith('t2_') ? t2Id : `t2_${t2Id}`;
+    const row = this.getByT2Stmt.get(normalized) as Record<string, unknown> | undefined;
+    return row ? this.mapRow(row) : null;
+  }
+
+  getByUsername(username: string): RedditUserRecord | null {
+    const row = this.getByUsernameStmt.get(username) as Record<string, unknown> | undefined;
+    return row ? this.mapRow(row) : null;
+  }
+
+  /** Bulk load — used to seed the in-memory cache on startup. */
+  getAll(): RedditUserRecord[] {
+    const rows = this.getAllStmt.all() as Record<string, unknown>[];
+    return rows.map((r) => this.mapRow(r));
+  }
+
+  private mapRow(row: Record<string, unknown>): RedditUserRecord {
+    return {
+      t2Id: row.t2_id as string,
+      username: row.username as string,
+      mxid: (row.mxid as string | null) ?? null,
+      updatedAt: row.updated_at as number,
+    };
+  }
+}