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/definition.py

@@ -3,7 +3,7 @@
 Inventory module definition.
 
 Defines the inventory module including its features, menu items,
-and route configurations.
+route configurations, and self-contained module settings.
 """
 
 from app.modules.base import ModuleDefinition
@@ -30,8 +30,9 @@ inventory_module = ModuleDefinition(
     name="Inventory Management",
     description=(
         "Stock level tracking, inventory locations, low stock alerts, "
-        "and product catalog management."
+        "transaction history, and bulk imports."
     ),
+    version="1.0.0",
     features=[
         "inventory_basic",  # Basic stock tracking
         "inventory_locations",  # Multiple warehouse locations
@@ -52,6 +53,14 @@ inventory_module = ModuleDefinition(
         ],
     },
     is_core=False,
+    # =========================================================================
+    # Self-Contained Module Configuration
+    # =========================================================================
+    is_self_contained=True,
+    services_path="app.modules.inventory.services",
+    models_path="app.modules.inventory.models",
+    schemas_path="app.modules.inventory.schemas",
+    exceptions_path="app.modules.inventory.exceptions",
 )