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/orders/models/__init__.py

@@ -2,15 +2,12 @@
 """
 Orders module database models.
 
-Re-exports order-related models from their source locations.
+This module contains the canonical implementations of order-related models.
 """
 
-from models.database.order import (
-    Order,
-    OrderItem,
-)
-from models.database.order_item_exception import OrderItemException
-from models.database.invoice import (
+from app.modules.orders.models.order import Order, OrderItem
+from app.modules.orders.models.order_item_exception import OrderItemException
+from app.modules.orders.models.invoice import (
     Invoice,
     InvoiceStatus,
     VATRegime,