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/payments/__init__.py

@@ -21,12 +21,20 @@ Menu Items:
 - Vendor: payment-methods (stored payment methods)
 """
 
-from app.modules.payments.definition import payments_module
+# Lazy imports to avoid circular dependencies
+
+__all__ = ["payments_module", "get_payments_module"]
 
 
 def get_payments_module():
     """Lazy getter to avoid circular imports."""
+    from app.modules.payments.definition import payments_module
     return payments_module
 
 
-__all__ = ["payments_module", "get_payments_module"]
+def __getattr__(name: str):
+    """Lazy import to avoid circular dependencies."""
+    if name == "payments_module":
+        from app.modules.payments.definition import payments_module
+        return payments_module
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")