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/services/__init__.py

@@ -0,0 +1,40 @@
+# app/modules/orders/services/__init__.py
+"""
+Orders module services.
+
+Re-exports order-related services from their source locations.
+"""
+
+from app.services.order_service import (
+    order_service,
+    OrderService,
+)
+from app.services.order_inventory_service import (
+    order_inventory_service,
+    OrderInventoryService,
+)
+from app.services.order_item_exception_service import (
+    order_item_exception_service,
+    OrderItemExceptionService,
+)
+from app.services.invoice_service import (
+    invoice_service,
+    InvoiceService,
+)
+from app.services.invoice_pdf_service import (
+    invoice_pdf_service,
+    InvoicePDFService,
+)
+
+__all__ = [
+    "order_service",
+    "OrderService",
+    "order_inventory_service",
+    "OrderInventoryService",
+    "order_item_exception_service",
+    "OrderItemExceptionService",
+    "invoice_service",
+    "InvoiceService",
+    "invoice_pdf_service",
+    "InvoicePDFService",
+]