feat: enhance calendar ICS generation with event type filtering

- Added support for filtering calendar events by type in the ICS generation API endpoint, allowing users to specify whether to include only duty shifts or all event types (duty, unavailable, vacation).
- Updated the `get_duties_for_user` function to accept an optional `event_types` parameter, enabling more flexible data retrieval based on user preferences.
- Enhanced unit tests to cover the new event type filtering functionality, ensuring correct behavior and reliability of the ICS generation process.

tests/test_run.py

@@ -1,7 +1,7 @@
 """Tests for duty_teller.run (main entry point with mocks)."""
 
 import asyncio
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import MagicMock, patch
 
 import pytest