feat: implement role-based access control for miniapp

- Introduced a new roles table in the database to manage user roles ('user' and 'admin') for access control.
- Updated the user model to include a foreign key reference to the roles table, allowing for role assignment.
- Enhanced command handlers to support the `/set_role` command for admins to assign roles to users.
- Refactored access control logic to utilize role checks instead of username/phone allowlists, improving security and maintainability.
- Updated documentation to reflect changes in access control mechanisms and role management.
- Added unit tests to ensure correct functionality of role assignment and access checks.

duty_teller/handlers/__init__.py

@@ -15,6 +15,7 @@ def register_handlers(app: Application) -> None:
     app.add_handler(commands.help_handler)
     app.add_handler(commands.set_phone_handler)
     app.add_handler(commands.calendar_link_handler)
+    app.add_handler(commands.set_role_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)