Refactor subscription handling and payment method management

- Updated the user router to import the subscription router directly, preparing for future package separation.
- Removed the outdated subscription handler file, streamlining the codebase and improving maintainability.
- Introduced a new method for deleting user payment methods by provider ID, enhancing flexibility in payment management.
- Improved error handling and clarity in payment method operations, ensuring a smoother user experience.

bot/handlers/user/subscription/__init__.py

@@ -0,0 +1,14 @@
+from aiogram import Router
+
+from . import core
+from . import payments
+from . import payment_methods
+
+router = Router(name="user_subscription_router")
+
+# Include sub-routers
+router.include_router(core.router)
+router.include_router(payments.router)
+router.include_router(payment_methods.router)
+
+