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/messaging/services/__init__.py

@@ -2,24 +2,29 @@
 """
 Messaging module services.
 
-Re-exports messaging-related services from their source locations.
+This module contains the canonical implementations of messaging-related services.
 """
 
-from app.services.messaging_service import (
+from app.modules.messaging.services.messaging_service import (
     messaging_service,
     MessagingService,
 )
-from app.services.message_attachment_service import (
+from app.modules.messaging.services.message_attachment_service import (
     message_attachment_service,
     MessageAttachmentService,
 )
-from app.services.admin_notification_service import (
+from app.modules.messaging.services.admin_notification_service import (
     admin_notification_service,
     AdminNotificationService,
+    platform_alert_service,
+    PlatformAlertService,
+    # Constants
+    NotificationType,
+    Priority,
+    AlertType,
+    Severity,
 )
 
-# Note: notification_service is a placeholder - not yet implemented
-
 __all__ = [
     "messaging_service",
     "MessagingService",
@@ -27,4 +32,11 @@ __all__ = [
     "MessageAttachmentService",
     "admin_notification_service",
     "AdminNotificationService",
+    "platform_alert_service",
+    "PlatformAlertService",
+    # Constants
+    "NotificationType",
+    "Priority",
+    "AlertType",
+    "Severity",
 ]