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/e2e/feed.py

@@ -772,7 +772,11 @@ def test_maintenance_mode_blocks_guests(alice):
     page, _ = alice
     try:
         _save_settings(page, maintenance_mode="1", maintenance_message="Down for tests")
+        deadline = time.time() + 5.0
         response = requests.get(f"{BASE_URL}/feed")
+        while response.status_code != 503 and time.time() < deadline:
+            time.sleep(0.2)
+            response = requests.get(f"{BASE_URL}/feed")
         assert response.status_code == 503
         assert "Down for tests" in response.text
     finally: