Add event type handling for duties in the system

- Introduced a new `event_type` column in the `duties` table to categorize duties as 'duty', 'unavailable', or 'vacation'.
- Updated the duty schedule import functionality to parse and store event types from the JSON input.
- Enhanced the API response to include event types for each duty, improving the calendar display logic.
- Modified the web application to visually differentiate between duty types in the calendar and duty list.
- Updated tests to cover new event type functionality and ensure correct parsing and storage of duties.
- Revised README documentation to reflect changes in duty event types and their representation in the system.

api/app.py

@@ -46,6 +46,7 @@ def _fetch_duties_response(from_date: str, to_date: str) -> list[DutyWithUser]:
                 start_at=duty.start_at,
                 end_at=duty.end_at,
                 full_name=full_name,
+                event_type=duty.event_type,
             )
             for duty, full_name in rows
         ]