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/handlers/start.py

@@ -24,7 +24,10 @@ async def cmd_start(message: Message) -> None:
     user_id = message.from_user.id
     chat_id = message.chat.id
 
-    await _check_and_prune_stale_topics(message, user_id, chat_id)
+    try:
+        await _check_and_prune_stale_topics(message, user_id, chat_id)
+    except Exception:
+        logger.exception("prune_stale_topics_error", user_id=user_id)
 
     active = db.get_active_chats(user_id)