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_group_duty_pin_service.py

@@ -103,6 +103,7 @@ class TestGetDutyMessageText:
     """Tests for get_duty_message_text."""
 
     def test_no_current_duty_returns_no_duty(self, session):
+        svc.duty_pin_cache.invalidate_pattern(("duty_message_text",))
         with patch(
             "duty_teller.services.group_duty_pin_service.get_current_duty",
             return_value=None,
@@ -113,6 +114,7 @@ class TestGetDutyMessageText:
         assert result == "No duty"
 
     def test_with_current_duty_returns_formatted(self, session, duty, user):
+        svc.duty_pin_cache.invalidate_pattern(("duty_message_text",))
         with patch(
             "duty_teller.services.group_duty_pin_service.get_current_duty",
             return_value=(duty, user),
@@ -130,6 +132,7 @@ class TestGetNextShiftEndUtc:
     """Tests for get_next_shift_end_utc."""
 
     def test_no_next_shift_returns_none(self, session):
+        svc.duty_pin_cache.invalidate(("next_shift_end",))
         with patch(
             "duty_teller.services.group_duty_pin_service.get_next_shift_end",
             return_value=None,
@@ -138,6 +141,7 @@ class TestGetNextShiftEndUtc:
         assert result is None
 
     def test_has_next_shift_returns_naive_utc(self, session):
+        svc.duty_pin_cache.invalidate(("next_shift_end",))
         naive = datetime(2025, 2, 21, 6, 0, 0)
         with patch(
             "duty_teller.services.group_duty_pin_service.get_next_shift_end",