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/services/letzshop/__init__.py

@@ -1,33 +1,33 @@
 # app/services/letzshop/__init__.py
 """
-Letzshop marketplace integration services.
+LEGACY LOCATION - Re-exports from module for backwards compatibility.
 
-Provides:
-- GraphQL client for API communication
-- Credential management service
-- Order import service
-- Fulfillment sync service
-- Vendor directory sync service
+The canonical implementation is now in:
+    app/modules/marketplace/services/letzshop/
+
+This file exists to maintain backwards compatibility with code that
+imports from the old location. All new code should import directly
+from the module:
+
+    from app.modules.marketplace.services.letzshop import LetzshopClient
 """
 
-from .client_service import (
-    LetzshopAPIError,
-    LetzshopAuthError,
+from app.modules.marketplace.services.letzshop import (
+    # Client
     LetzshopClient,
     LetzshopClientError,
+    LetzshopAuthError,
+    LetzshopAPIError,
     LetzshopConnectionError,
-)
-from .credentials_service import (
+    # Credentials
+    LetzshopCredentialsService,
     CredentialsError,
     CredentialsNotFoundError,
-    LetzshopCredentialsService,
-)
-from .order_service import (
+    # Order Service
     LetzshopOrderService,
     OrderNotFoundError,
     VendorNotFoundError,
-)
-from .vendor_sync_service import (
+    # Vendor Sync Service
     LetzshopVendorSyncService,
     get_vendor_sync_service,
 )