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/exceptions.py

@@ -0,0 +1,53 @@
+# app/modules/orders/exceptions.py
+"""
+Orders module exceptions.
+
+Re-exports order-related exceptions from their source locations.
+"""
+
+from app.exceptions.order import (
+    OrderNotFoundException,
+    OrderAlreadyExistsException,
+    OrderValidationException,
+    InvalidOrderStatusException,
+    OrderCannotBeCancelledException,
+)
+
+from app.exceptions.order_item_exception import (
+    OrderItemExceptionNotFoundException,
+    OrderHasUnresolvedExceptionsException,
+    ExceptionAlreadyResolvedException,
+    InvalidProductForExceptionException,
+)
+
+from app.exceptions.invoice import (
+    InvoiceNotFoundException,
+    InvoiceSettingsNotFoundException,
+    InvoiceSettingsAlreadyExistException,
+    InvoiceValidationException,
+    InvoicePDFGenerationException,
+    InvoicePDFNotFoundException,
+    InvalidInvoiceStatusTransitionException,
+)
+
+__all__ = [
+    # Order exceptions
+    "OrderNotFoundException",
+    "OrderAlreadyExistsException",
+    "OrderValidationException",
+    "InvalidOrderStatusException",
+    "OrderCannotBeCancelledException",
+    # Order item exception exceptions
+    "OrderItemExceptionNotFoundException",
+    "OrderHasUnresolvedExceptionsException",
+    "ExceptionAlreadyResolvedException",
+    "InvalidProductForExceptionException",
+    # Invoice exceptions
+    "InvoiceNotFoundException",
+    "InvoiceSettingsNotFoundException",
+    "InvoiceSettingsAlreadyExistException",
+    "InvoiceValidationException",
+    "InvoicePDFGenerationException",
+    "InvoicePDFNotFoundException",
+    "InvalidInvoiceStatusTransitionException",
+]