feat: implement caching for duty-related data and enhance performance

- Added a TTLCache class for in-memory caching of duty-related data, improving performance by reducing database queries.
- Integrated caching into the group duty pin functionality, allowing for efficient retrieval of message text and next shift end times.
- Introduced new methods to invalidate caches when relevant data changes, ensuring data consistency.
- Created a new Alembic migration to add indexes on the duties table for improved query performance.
- Updated tests to cover the new caching behavior and ensure proper functionality.

tests/test_app.py

@@ -403,6 +403,9 @@ def test_calendar_ical_ignores_unknown_query_params(
     """Unknown query params (e.g. events=all) are ignored; response is duty-only."""
     from types import SimpleNamespace
 
+    from duty_teller.cache import ics_calendar_cache
+
+    ics_calendar_cache.invalidate(("personal_ics", 1))
     mock_user = SimpleNamespace(id=1, full_name="User A")
     mock_get_user.return_value = mock_user
     duty = SimpleNamespace(