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.

db/models.py

@@ -32,7 +32,8 @@ class Duty(Base):
     user_id: Mapped[int] = mapped_column(
         Integer, ForeignKey("users.id"), nullable=False
     )
-    start_at: Mapped[str] = mapped_column(Text, nullable=False)  # ISO 8601
-    end_at: Mapped[str] = mapped_column(Text, nullable=False)  # ISO 8601
+    # 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)
 
     user: Mapped["User"] = relationship("User", back_populates="duties")