refactor: move billingWebhookHandler to serviceBilling layer

Business logic for Stripe webhooks belongs in serviceCenter/services/serviceBilling/, not in routes/. Updates 3 lazy imports in routeBilling.py accordingly.

Co-authored-by: Cursor <cursoragent@cursor.com>

modules/routes/routeBilling.py

@@ -332,7 +332,7 @@ def _getStripeClient():
 
 def _creditStripeSessionIfNeeded(billingInterface, session: Dict[str, Any], eventId: Optional[str] = None) -> CheckoutConfirmResponse:
     """Credit balance from Stripe Checkout session if not already credited."""
-    from .billingWebhookHandler import creditStripeSessionIfNeeded
+    from modules.serviceCenter.services.serviceBilling.billingWebhookHandler import creditStripeSessionIfNeeded
     return creditStripeSessionIfNeeded(billingInterface, session, eventId, CheckoutConfirmResponse)
 
 
@@ -1079,13 +1079,13 @@ async def stripeWebhook(
 
 def handleSubscriptionCheckoutCompleted(session, eventId: str) -> None:
     """Handle checkout.session.completed for mode=subscription."""
-    from .billingWebhookHandler import handleSubscriptionCheckoutCompleted as _handler
+    from modules.serviceCenter.services.serviceBilling.billingWebhookHandler import handleSubscriptionCheckoutCompleted as _handler
     _handler(session, eventId, getRootInterface)
 
 
 def _handleSubscriptionWebhook(event) -> None:
     """Process Stripe subscription webhook events."""
-    from .billingWebhookHandler import handleSubscriptionWebhook as _handler
+    from modules.serviceCenter.services.serviceBilling.billingWebhookHandler import handleSubscriptionWebhook as _handler
     _handler(event, getRootInterface)
 
 

modules/serviceCenter/services/serviceBilling/billingWebhookHandler.py

@@ -2,7 +2,7 @@
 # All rights reserved.
 """
 Stripe webhook and subscription business logic for billing.
-Extracted from routeBilling.py for maintainability.
+Handles checkout credit, subscription lifecycle transitions, and invoice events.
 """
 
 import logging