Add subscription link field to user model

- Add subscription_link column to users table (nullable, unique)
- Add SUBSCRIPTION_URL to config settings
- Update user creation to include subscription link from Remnawave
- Add support subscription ticket creation with formatted message
- Add "update link" button to main menu
- Refactor support subscription formatting to include user details

states/buy.py

@@ -4,3 +4,4 @@ from aiogram.fsm.state import StatesGroup, State
 class SubscriptionStorage(StatesGroup):
     devices = State()
     duration = State()
+    checkout = State()