feat: implement billing system with Pally integration

- Added billing models and repository for handling bills.
- Introduced BillingService to manage payment initiation and bill creation.
- Updated user service to remove ticket-related functionality.
- Refactored settings to include new billing-related fields.
- Removed ticket-related handlers and schemas.
- Added new billing handlers for processing payments.
- Updated database schema with new bills table and status enum.
- Enhanced utility functions to calculate prices considering discounts.
- Introduced prehandling for non-private messages.
- Updated main application to initialize new billing services and repositories.

db/models/__init__.py

@@ -1,4 +1,4 @@
-from .tickets import Ticket
+from .bills import Bill
 from .users import User
 
-__all__ = ["Ticket", "User"]
+__all__ = ["Bill", "User"]