feat: add calendar subscription token functionality and ICS generation

- Introduced a new database model for calendar subscription tokens, allowing users to generate unique tokens for accessing their personal calendar.
- Implemented API endpoint to return ICS files containing only the subscribing user's duties, enhancing user experience with personalized calendar access.
- Added utility functions for generating ICS files from user duties, ensuring proper formatting and timezone handling.
- Updated command handlers to support the new calendar link feature, providing users with easy access to their personal calendar subscriptions.
- Included unit tests for the new functionality, ensuring reliability and correctness of token generation and ICS file creation.

duty_teller/handlers/__init__.py

@@ -9,6 +9,7 @@ def register_handlers(app: Application) -> None:
     app.add_handler(commands.start_handler)
     app.add_handler(commands.help_handler)
     app.add_handler(commands.set_phone_handler)
+    app.add_handler(commands.calendar_link_handler)
     app.add_handler(import_duty_schedule.import_duty_schedule_handler)
     app.add_handler(import_duty_schedule.handover_time_handler)
     app.add_handler(import_duty_schedule.duty_schedule_document_handler)