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/inventory/services/__init__.py

@@ -2,20 +2,21 @@
 """
 Inventory module services.
 
-Re-exports inventory-related services from their source locations.
+This module contains the canonical implementations of inventory-related services.
 """
 
-from app.services.inventory_service import (
+from app.modules.inventory.services.inventory_service import (
     inventory_service,
     InventoryService,
 )
-from app.services.inventory_transaction_service import (
+from app.modules.inventory.services.inventory_transaction_service import (
     inventory_transaction_service,
     InventoryTransactionService,
 )
-from app.services.inventory_import_service import (
+from app.modules.inventory.services.inventory_import_service import (
     inventory_import_service,
     InventoryImportService,
+    ImportResult,
 )
 
 __all__ = [
@@ -25,4 +26,5 @@ __all__ = [
     "InventoryTransactionService",
     "inventory_import_service",
     "InventoryImportService",
+    "ImportResult",
 ]