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/schemas.py

@@ -23,8 +23,8 @@ class UserInDb(UserBase):
 
 class DutyBase(BaseModel):
     user_id: int
-    start_at: str  # ISO 8601
-    end_at: str  # ISO 8601
+    start_at: str  # UTC, ISO 8601 with Z
+    end_at: str  # UTC, ISO 8601 with Z
 
 
 class DutyCreate(DutyBase):
@@ -43,3 +43,10 @@ class DutyWithUser(DutyInDb):
     full_name: str
 
     model_config = ConfigDict(from_attributes=True)
+
+
+class CalendarEvent(BaseModel):
+    """External calendar event (e.g. holiday) for a single day."""
+
+    date: str  # YYYY-MM-DD
+    summary: str