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/routes/api/admin.py

@@ -17,8 +17,7 @@ from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_admin_api, require_module_access
 from app.core.database import get_db
-from app.services.admin_subscription_service import admin_subscription_service
-from app.services.subscription_service import subscription_service
+from app.modules.billing.services import admin_subscription_service, subscription_service
 from models.database.user import User
 from models.schema.billing import (
     BillingHistoryListResponse,