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/messaging/routes/api/vendor_email_settings.py

@@ -20,7 +20,7 @@ from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_vendor_api
 from app.core.database import get_db
-from app.modules.cms.services.vendor_email_settings_service import VendorEmailSettingsService
+from app.modules.cms.services.vendor_email_settings_service import vendor_email_settings_service
 from app.modules.billing.services.subscription_service import subscription_service
 from models.schema.auth import UserContext
 
@@ -137,9 +137,8 @@ def get_email_settings(
     Returns settings with sensitive fields masked.
     """
     vendor_id = current_user.token_vendor_id
-    service = VendorEmailSettingsService(db)
 
-    settings = service.get_settings(vendor_id)
+    settings = vendor_email_settings_service.get_settings(db, vendor_id)
     if not settings:
         return EmailSettingsResponse(
             configured=False,
@@ -165,8 +164,7 @@ def get_email_status(
     Used by frontend to show warning banner if not configured.
     """
     vendor_id = current_user.token_vendor_id
-    service = VendorEmailSettingsService(db)
-    status = service.get_status(vendor_id)
+    status = vendor_email_settings_service.get_status(db, vendor_id)
     return EmailStatusResponse(**status)
 
 
@@ -181,13 +179,12 @@ def get_available_providers(
     Returns list of providers with availability status.
     """
     vendor_id = current_user.token_vendor_id
-    service = VendorEmailSettingsService(db)
 
     # Get vendor's current tier
     tier = subscription_service.get_current_tier(db, vendor_id)
 
     return ProvidersResponse(
-        providers=service.get_available_providers(tier),
+        providers=vendor_email_settings_service.get_available_providers(tier),
         current_tier=tier.value if tier else None,
     )
 
@@ -206,13 +203,13 @@ def update_email_settings(
     Raises ValidationException if data is invalid.
     """
     vendor_id = current_user.token_vendor_id
-    service = VendorEmailSettingsService(db)
 
     # Get vendor's current tier for validation
     tier = subscription_service.get_current_tier(db, vendor_id)
 
     # Service raises appropriate exceptions (API-003 compliance)
-    settings = service.create_or_update(
+    settings = vendor_email_settings_service.create_or_update(
+        db=db,
         vendor_id=vendor_id,
         data=data.model_dump(exclude_unset=True),
         current_tier=tier,
@@ -240,10 +237,9 @@ def verify_email_settings(
     Raises ValidationException if verification fails.
     """
     vendor_id = current_user.token_vendor_id
-    service = VendorEmailSettingsService(db)
 
     # Service raises appropriate exceptions (API-003 compliance)
-    result = service.verify_settings(vendor_id, data.test_email)
+    result = vendor_email_settings_service.verify_settings(db, vendor_id, data.test_email)
     db.commit()
 
     return EmailVerifyResponse(
@@ -264,10 +260,9 @@ def delete_email_settings(
     Raises ResourceNotFoundException if settings not found.
     """
     vendor_id = current_user.token_vendor_id
-    service = VendorEmailSettingsService(db)
 
     # Service raises ResourceNotFoundException if not found (API-003 compliance)
-    service.delete(vendor_id)
+    vendor_email_settings_service.delete(db, vendor_id)
     db.commit()
 
     return EmailDeleteResponse(