feat: implement subscription management and billing enhancements

repositories/users.py

@@ -105,7 +105,7 @@ class UserRepository:
         if not user or user.balance < amount:
             return None
 
-        created_at = datetime.datetime.now(datetime.timezone.UTC)
+        created_at = datetime.datetime.now(datetime.UTC)
         transaction = BalanceTransaction(
             user_id=user_id,
             amount=amount,