refactor: standardize modular architecture patterns

- Rename module definition variables to follow naming convention:
  - catalog/definition.py: module → catalog_module
  - checkout/definition.py: module → checkout_module
  - cart/definition.py: module → cart_module

- Add router attachment functions for lazy loading:
  - get_catalog_module_with_routers()
  - get_checkout_module_with_routers()
  - get_cart_module_with_routers()

- Move billing exceptions to exceptions.py:
  - Add backwards-compatible aliases (BillingServiceError, etc.)
  - Update billing_service.py to import from exceptions.py

- Standardize VendorEmailSettingsService DI pattern:
  - Change from db in __init__ to db as method parameter
  - Create singleton vendor_email_settings_service instance
  - Update routes and tests to use new pattern

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/billing/exceptions.py

@@ -19,10 +19,13 @@ from app.exceptions.base import (
 __all__ = [
     # Base billing exception
     "BillingException",
+    "BillingServiceError",  # Alias for backwards compatibility
     # Subscription exceptions
     "SubscriptionNotFoundException",
     "NoActiveSubscriptionException",
+    "NoActiveSubscriptionError",  # Alias for backwards compatibility
     "SubscriptionNotCancelledException",
+    "SubscriptionNotCancelledError",  # Alias for backwards compatibility
     "SubscriptionAlreadyCancelledException",
     # Tier exceptions
     "TierNotFoundException",
@@ -30,8 +33,10 @@ __all__ = [
     "TierLimitExceededException",
     # Payment exceptions
     "PaymentSystemNotConfiguredException",
+    "PaymentSystemNotConfiguredError",  # Alias for backwards compatibility
     "StripeNotConfiguredException",
     "StripePriceNotConfiguredException",
+    "StripePriceNotConfiguredError",  # Alias for backwards compatibility
     "PaymentFailedException",
     # Webhook exceptions
     "InvalidWebhookSignatureException",
@@ -57,6 +62,10 @@ class BillingException(BusinessLogicException):
         super().__init__(message=message, error_code=error_code, details=details)
 
 
+# Alias for backwards compatibility with billing_service.py
+BillingServiceError = BillingException
+
+
 # =============================================================================
 # Subscription Exceptions
 # =============================================================================
@@ -83,6 +92,10 @@ class NoActiveSubscriptionException(BusinessLogicException):
         )
 
 
+# Alias for backwards compatibility with billing_service.py
+NoActiveSubscriptionError = NoActiveSubscriptionException
+
+
 class SubscriptionNotCancelledException(BusinessLogicException):
     """Raised when trying to reactivate a subscription that is not cancelled."""
 
@@ -93,6 +106,10 @@ class SubscriptionNotCancelledException(BusinessLogicException):
         )
 
 
+# Alias for backwards compatibility with billing_service.py
+SubscriptionNotCancelledError = SubscriptionNotCancelledException
+
+
 class SubscriptionAlreadyCancelledException(BusinessLogicException):
     """Raised when trying to cancel an already cancelled subscription."""
 
@@ -163,6 +180,10 @@ class PaymentSystemNotConfiguredException(ServiceUnavailableException):
         super().__init__(message="Payment system not configured")
 
 
+# Alias for backwards compatibility with billing_service.py
+PaymentSystemNotConfiguredError = PaymentSystemNotConfiguredException
+
+
 class StripeNotConfiguredException(BillingException):
     """Raised when Stripe is not configured."""
 
@@ -185,6 +206,10 @@ class StripePriceNotConfiguredException(BusinessLogicException):
         self.tier_code = tier_code
 
 
+# Alias for backwards compatibility with billing_service.py
+StripePriceNotConfiguredError = StripePriceNotConfiguredException
+
+
 class PaymentFailedException(BillingException):
     """Raised when a payment fails."""