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/__init__.py

@@ -2,22 +2,35 @@
 """
 Inventory Module - Stock and product management.
 
-This module provides:
+This is a self-contained module providing:
 - Inventory tracking across locations
 - Stock level management
 - Low stock alerts
 - Inventory transactions and history
-- Product catalog management
+- Bulk inventory imports
 
-Routes:
-- Admin: /api/v1/admin/inventory/*
-- Vendor: /api/v1/vendor/inventory/*
-
-Menu Items:
-- Admin: inventory, vendor-products
-- Vendor: products, inventory
+Module Structure:
+- models/ - Database models (Inventory, InventoryTransaction)
+- services/ - Business logic (InventoryService, InventoryTransactionService)
+- schemas/ - Pydantic DTOs
+- routes/ - API routes
+- exceptions.py - Module-specific exceptions
 """
 
-from app.modules.inventory.definition import inventory_module
+# Use lazy imports to avoid circular import issues
 
-__all__ = ["inventory_module"]
+
+def __getattr__(name: str):
+    """Lazy import module components to avoid circular imports."""
+    if name == "inventory_module":
+        from app.modules.inventory.definition import inventory_module
+
+        return inventory_module
+    elif name == "get_inventory_module_with_routers":
+        from app.modules.inventory.definition import get_inventory_module_with_routers
+
+        return get_inventory_module_with_routers
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+
+
+__all__ = ["inventory_module", "get_inventory_module_with_routers"]