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/customers/models/__init__.py

@@ -2,14 +2,23 @@
 """
 Customers module database models.
 
-Re-exports customer-related models from their source locations.
+This is the canonical location for customer models. Module models are
+automatically discovered and registered with SQLAlchemy's Base.metadata
+at startup.
+
+Usage:
+    from app.modules.customers.models import (
+        Customer,
+        CustomerAddress,
+        PasswordResetToken,
+    )
 """
 
-from models.database.customer import (
+from app.modules.customers.models.customer import (
     Customer,
     CustomerAddress,
 )
-from models.database.password_reset_token import PasswordResetToken
+from app.modules.customers.models.password_reset_token import PasswordResetToken
 
 __all__ = [
     "Customer",