Implement duty schedule import functionality and enhance user management

- Added a new command `/import_duty_schedule` for importing duty schedules via JSON, restricted to admin users.
- Introduced a two-step import process: specifying handover time and uploading a JSON file.
- Updated the database schema to allow `telegram_user_id` to be nullable for user creation by full name.
- Implemented repository functions for user management, including `get_or_create_user_by_full_name` and `delete_duties_in_range`.
- Enhanced README documentation with details on the new import command and JSON format requirements.
- Added comprehensive tests for the duty schedule parser and integration tests for the import functionality.

handlers/__init__.py

@@ -2,10 +2,13 @@
 
 from telegram.ext import Application
 
-from . import commands, errors
+from . import commands, errors, import_duty_schedule
 
 
 def register_handlers(app: Application) -> None:
     app.add_handler(commands.start_handler)
     app.add_handler(commands.help_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)
     app.add_error_handler(errors.error_handler)