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/services/__init__.py

@@ -20,6 +20,8 @@ from app.modules.billing.services.admin_subscription_service import (
 from app.modules.billing.services.billing_service import (
     BillingService,
     billing_service,
+)
+from app.modules.billing.exceptions import (
     BillingServiceError,
     PaymentSystemNotConfiguredError,
     TierNotFoundError,

app/modules/billing/services/billing_service.py

@@ -23,54 +23,19 @@ from app.modules.billing.models import (
     VendorAddOn,
     VendorSubscription,
 )
+from app.modules.billing.exceptions import (
+    BillingServiceError,
+    NoActiveSubscriptionError,
+    PaymentSystemNotConfiguredError,
+    StripePriceNotConfiguredError,
+    SubscriptionNotCancelledError,
+    TierNotFoundError,
+)
 from app.modules.tenancy.models import Vendor
 
 logger = logging.getLogger(__name__)
 
 
-class BillingServiceError(Exception):
-    """Base exception for billing service errors."""
-
-    pass
-
-
-class PaymentSystemNotConfiguredError(BillingServiceError):
-    """Raised when Stripe is not configured."""
-
-    def __init__(self):
-        super().__init__("Payment system not configured")
-
-
-class TierNotFoundError(BillingServiceError):
-    """Raised when a tier is not found."""
-
-    def __init__(self, tier_code: str):
-        self.tier_code = tier_code
-        super().__init__(f"Tier '{tier_code}' not found")
-
-
-class StripePriceNotConfiguredError(BillingServiceError):
-    """Raised when Stripe price is not configured for a tier."""
-
-    def __init__(self, tier_code: str):
-        self.tier_code = tier_code
-        super().__init__(f"Stripe price not configured for tier '{tier_code}'")
-
-
-class NoActiveSubscriptionError(BillingServiceError):
-    """Raised when no active subscription exists."""
-
-    def __init__(self):
-        super().__init__("No active subscription found")
-
-
-class SubscriptionNotCancelledError(BillingServiceError):
-    """Raised when trying to reactivate a non-cancelled subscription."""
-
-    def __init__(self):
-        super().__init__("Subscription is not cancelled")
-
-
 class BillingService:
     """Service for billing operations."""