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.

config.py

@@ -41,6 +41,9 @@ CORS_ORIGINS = (
     else ["*"]
 )
 
+# Optional: URL of a public ICS calendar (e.g. holidays). Empty = no external calendar; /api/calendar-events returns [].
+EXTERNAL_CALENDAR_ICS_URL = os.getenv("EXTERNAL_CALENDAR_ICS_URL", "").strip()
+
 
 def is_admin(username: str) -> bool:
     """True if the given Telegram username (no @, any case) is in ADMIN_USERNAMES."""