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/cms/services/__init__.py

@@ -19,7 +19,8 @@ from app.modules.cms.services.vendor_theme_service import (
 )
 from app.modules.cms.services.vendor_email_settings_service import (
     VendorEmailSettingsService,
-    get_vendor_email_settings_service,
+    vendor_email_settings_service,
+    get_vendor_email_settings_service,  # Deprecated: use vendor_email_settings_service
 )
 
 __all__ = [
@@ -30,5 +31,6 @@ __all__ = [
     "VendorThemeService",
     "vendor_theme_service",
     "VendorEmailSettingsService",
-    "get_vendor_email_settings_service",
+    "vendor_email_settings_service",
+    "get_vendor_email_settings_service",  # Deprecated
 ]