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/inventory/exceptions.py

@@ -0,0 +1,26 @@
+# app/modules/inventory/exceptions.py
+"""
+Inventory module exceptions.
+
+Re-exports inventory-related exceptions from their source locations.
+"""
+
+from app.exceptions.inventory import (
+    InventoryNotFoundException,
+    InsufficientInventoryException,
+    InvalidInventoryOperationException,
+    InventoryValidationException,
+    NegativeInventoryException,
+    InvalidQuantityException,
+    LocationNotFoundException,
+)
+
+__all__ = [
+    "InventoryNotFoundException",
+    "InsufficientInventoryException",
+    "InvalidInventoryOperationException",
+    "InventoryValidationException",
+    "NegativeInventoryException",
+    "InvalidQuantityException",
+    "LocationNotFoundException",
+]