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/orders/models/__init__.py

@@ -0,0 +1,28 @@
+# app/modules/orders/models/__init__.py
+"""
+Orders module database models.
+
+Re-exports order-related models from their source locations.
+"""
+
+from models.database.order import (
+    Order,
+    OrderItem,
+)
+from models.database.order_item_exception import OrderItemException
+from models.database.invoice import (
+    Invoice,
+    InvoiceStatus,
+    VATRegime,
+    VendorInvoiceSettings,
+)
+
+__all__ = [
+    "Order",
+    "OrderItem",
+    "OrderItemException",
+    "Invoice",
+    "InvoiceStatus",
+    "VATRegime",
+    "VendorInvoiceSettings",
+]