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/exceptions.py

@@ -0,0 +1,28 @@
+# app/modules/messaging/exceptions.py
+"""
+Messaging module exceptions.
+
+Re-exports messaging-related exceptions from their source locations.
+"""
+
+from app.exceptions.message import (
+    ConversationNotFoundException,
+    MessageNotFoundException,
+    ConversationClosedException,
+    MessageAttachmentException,
+    UnauthorizedConversationAccessException,
+    InvalidConversationTypeException,
+    InvalidRecipientTypeException,
+    AttachmentNotFoundException,
+)
+
+__all__ = [
+    "ConversationNotFoundException",
+    "MessageNotFoundException",
+    "ConversationClosedException",
+    "MessageAttachmentException",
+    "UnauthorizedConversationAccessException",
+    "InvalidConversationTypeException",
+    "InvalidRecipientTypeException",
+    "AttachmentNotFoundException",
+]