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/customers/exceptions.py

@@ -0,0 +1,37 @@
+# app/modules/customers/exceptions.py
+"""
+Customers module exceptions.
+
+Re-exports customer-related exceptions from their source locations.
+"""
+
+from app.exceptions.customer import (
+    CustomerNotFoundException,
+    CustomerAlreadyExistsException,
+    DuplicateCustomerEmailException,
+    CustomerNotActiveException,
+    InvalidCustomerCredentialsException,
+    CustomerValidationException,
+    CustomerAuthorizationException,
+)
+
+from app.exceptions.address import (
+    AddressNotFoundException,
+    AddressLimitExceededException,
+    InvalidAddressTypeException,
+)
+
+__all__ = [
+    # Customer exceptions
+    "CustomerNotFoundException",
+    "CustomerAlreadyExistsException",
+    "DuplicateCustomerEmailException",
+    "CustomerNotActiveException",
+    "InvalidCustomerCredentialsException",
+    "CustomerValidationException",
+    "CustomerAuthorizationException",
+    # Address exceptions
+    "AddressNotFoundException",
+    "AddressLimitExceededException",
+    "InvalidAddressTypeException",
+]