fix(tg): reviewer fixes — error handling, timeouts, db index

- commands.py: try/except TelegramBadRequest around all Bot API calls (#2);
  /new handles "topics limit" with user-friendly message (#4)
- start.py: isolate _check_and_prune_stale_topics with try/except Exception (#3)
- message.py: asyncio.timeout(30) around stream_message; handle TimeoutError (#6)
- db.py: add idx_chats_user_id index in init_db() (#7)
- settings.py: remove dead active_chat_id variable (#8)
- tests: add test_message.py (stream error/success); add 2 tests in test_commands.py
  (topics limit, /archive in General topic)

adapter/telegram/db.py

@@ -29,6 +29,7 @@ def init_db() -> None:
                 created_at  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
                 PRIMARY KEY (user_id, thread_id)
             );
+            CREATE INDEX IF NOT EXISTS idx_chats_user_id ON chats(user_id);
         """)