Implement external calendar integration and enhance API functionality

- Added support for fetching and parsing external ICS calendars, allowing events to be displayed on the duty grid.
- Introduced a new API endpoint `/api/calendar-events` to retrieve calendar events within a specified date range.
- Updated configuration to include `EXTERNAL_CALENDAR_ICS_URL` for specifying the ICS calendar URL.
- Enhanced the web application to visually indicate days with events and provide event summaries on hover.
- Improved documentation in the README to include details about the new calendar integration and configuration options.
- Updated tests to cover the new calendar functionality and ensure proper integration.

tests/test_app.py

@@ -92,8 +92,8 @@ def test_duties_200_with_allowed_user(mock_can_access, mock_validate, client):
             {
                 "id": 1,
                 "user_id": 10,
-                "start_at": "2025-01-15T09:00:00",
-                "end_at": "2025-01-15T18:00:00",
+                "start_at": "2025-01-15T09:00:00Z",
+                "end_at": "2025-01-15T18:00:00Z",
                 "full_name": "Иван Иванов",
             }
         ]