refactor: migrate modules from re-exports to canonical implementations

Move actual code implementations into module directories:
- orders: 5 services, 4 models, order/invoice schemas
- inventory: 3 services, 2 models, 30+ schemas
- customers: 3 services, 2 models, customer schemas
- messaging: 3 services, 2 models, message/notification schemas
- monitoring: background_tasks_service
- marketplace: 5+ services including letzshop submodule
- dev_tools: code_quality_service, test_runner_service
- billing: billing_service
- contracts: definition.py

Legacy files in app/services/, models/database/, models/schema/
now re-export from canonical module locations for backwards
compatibility. Architecture validator passes with 0 errors.

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

app/modules/billing/services/__init__.py

@@ -17,6 +17,16 @@ from app.modules.billing.services.admin_subscription_service import (
     AdminSubscriptionService,
     admin_subscription_service,
 )
+from app.modules.billing.services.billing_service import (
+    BillingService,
+    billing_service,
+    BillingServiceError,
+    PaymentSystemNotConfiguredError,
+    TierNotFoundError,
+    StripePriceNotConfiguredError,
+    NoActiveSubscriptionError,
+    SubscriptionNotCancelledError,
+)
 
 __all__ = [
     "SubscriptionService",
@@ -25,4 +35,12 @@ __all__ = [
     "stripe_service",
     "AdminSubscriptionService",
     "admin_subscription_service",
+    "BillingService",
+    "billing_service",
+    "BillingServiceError",
+    "PaymentSystemNotConfiguredError",
+    "TierNotFoundError",
+    "StripePriceNotConfiguredError",
+    "NoActiveSubscriptionError",
+    "SubscriptionNotCancelledError",
 ]