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/schemas/__init__.py

@@ -0,0 +1,28 @@
+# app/modules/messaging/schemas/__init__.py
+"""
+Messaging module Pydantic schemas.
+
+Re-exports messaging-related schemas from their source locations.
+"""
+
+from models.schema.message import (
+    ConversationResponse,
+    ConversationListResponse,
+    MessageResponse,
+    MessageCreate,
+    AttachmentResponse,
+)
+from models.schema.notification import (
+    NotificationResponse,
+    NotificationListResponse,
+)
+
+__all__ = [
+    "ConversationResponse",
+    "ConversationListResponse",
+    "MessageResponse",
+    "MessageCreate",
+    "AttachmentResponse",
+    "NotificationResponse",
+    "NotificationListResponse",
+]