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.

db/models.py

@@ -35,5 +35,7 @@ class Duty(Base):
     # UTC, ISO 8601 with Z suffix (e.g. 2025-01-15T09:00:00Z)
     start_at: Mapped[str] = mapped_column(Text, nullable=False)
     end_at: Mapped[str] = mapped_column(Text, nullable=False)
+    # duty | unavailable | vacation
+    event_type: Mapped[str] = mapped_column(Text, nullable=False, server_default="duty")
 
     user: Mapped["User"] = relationship("User", back_populates="duties")