fix: clear rate-limit hot-settings cache and add retry loop for maintenance-mode tests

- Invalidate `_hot_settings_value` and `_hot_settings_at` in `_patch_limits` so stale
  cached limits from prior tests do not persist after monkey-patching `get_int_setting`.
- Add a polling retry loop (up to 5 seconds) in `test_maintenance_mode_blocks_guests`
  to handle eventual consistency of the maintenance-mode flag propagation.
- Replace brittle hardcoded `@bob_test` assertion in mention test with dynamic
  `data-username` attribute extraction from the active dropdown item.
- Replace `asyncio.run` calls in bot unit tests with shared `run_async` helper from
  `tests.conftest` to avoid event-loop conflicts in the test suite.
- Change `insert` to `upsert` with `["uid"]` conflict target in Gitea test fixture
  to prevent duplicate-key errors on repeated test runs.
- Replace `asyncio.new_event_loop().run_until_complete` in Telegram worker tests
  with the same shared `run_async` helper for consistency.

tests/unit/services/telegram/worker.py

@@ -1,13 +1,12 @@
 # retoor <retoor@molodetz.nl>
 
-import asyncio
-
 from devplacepy.services.telegram.backend import FakeTelegramBackend
 from devplacepy.services.telegram.worker import TelegramWorker
+from tests.conftest import run_async
 
 
 def _run(coro):
-    return asyncio.new_event_loop().run_until_complete(coro)
+    return run_async(coro)
 
 
 def _worker(backend):