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/services/__init__.py

@@ -0,0 +1,28 @@
+# app/modules/inventory/services/__init__.py
+"""
+Inventory module services.
+
+Re-exports inventory-related services from their source locations.
+"""
+
+from app.services.inventory_service import (
+    inventory_service,
+    InventoryService,
+)
+from app.services.inventory_transaction_service import (
+    inventory_transaction_service,
+    InventoryTransactionService,
+)
+from app.services.inventory_import_service import (
+    inventory_import_service,
+    InventoryImportService,
+)
+
+__all__ = [
+    "inventory_service",
+    "InventoryService",
+    "inventory_transaction_service",
+    "InventoryTransactionService",
+    "inventory_import_service",
+    "InventoryImportService",
+]