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.

.env.example

@@ -9,3 +9,6 @@ ADMIN_USERNAMES=admin1,admin2
 
 # Dev only: set to 1 to allow calendar without Telegram initData (insecure; do not use in production).
 # MINI_APP_SKIP_AUTH=1
+
+# Optional: URL of a public ICS calendar (e.g. holidays). Days from this calendar are highlighted on the duty grid; click "i" for summary.
+# EXTERNAL_CALENDAR_ICS_URL=https://example.com/holidays.ics