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/gitea.py

@@ -94,7 +94,7 @@ def _make_user_issues_gitea():
     _counter_issues_gitea[0] += 1
     uid = f"issuetest-user-{_counter_issues_gitea[0]}"
     username = f"issuetester{_counter_issues_gitea[0]}"
-    get_table("users").insert(
+    get_table("users").upsert(
         {
             "uid": uid,
             "username": username,
@@ -102,7 +102,8 @@ def _make_user_issues_gitea():
             "xp": 0,
             "level": 1,
             "is_active": True,
-        }
+        },
+        ["uid"],
     )
     return uid, username
 def _unread(user_uid):