feat: add self-contained structure to remaining modules

Add exceptions, models, schemas, services directories to modules:

customers:
- exceptions.py, models/, schemas/, services/

inventory:
- exceptions.py, models/, schemas/, services/

messaging:
- exceptions.py, models/, schemas/, services/

monitoring:
- exceptions.py, models/, schemas/, services/

orders:
- exceptions.py, models/, schemas/, services/

payments:
- Updated __init__.py

All modules now have the standard self-contained directory
structure ready for future migration of business logic.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/messaging/services/__init__.py

@@ -0,0 +1,30 @@
+# app/modules/messaging/services/__init__.py
+"""
+Messaging module services.
+
+Re-exports messaging-related services from their source locations.
+"""
+
+from app.services.messaging_service import (
+    messaging_service,
+    MessagingService,
+)
+from app.services.message_attachment_service import (
+    message_attachment_service,
+    MessageAttachmentService,
+)
+from app.services.admin_notification_service import (
+    admin_notification_service,
+    AdminNotificationService,
+)
+
+# Note: notification_service is a placeholder - not yet implemented
+
+__all__ = [
+    "messaging_service",
+    "MessagingService",
+    "message_attachment_service",
+    "MessageAttachmentService",
+    "admin_notification_service",
+    "AdminNotificationService",
+]