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/schemas/__init__.py

@@ -0,0 +1,28 @@
+# app/modules/inventory/schemas/__init__.py
+"""
+Inventory module Pydantic schemas.
+
+Re-exports inventory-related schemas from their source locations.
+"""
+
+from models.schema.inventory import (
+    InventoryCreate,
+    InventoryAdjust,
+    InventoryReserve,
+    InventoryResponse,
+    InventoryListResponse,
+    InventoryTransactionResponse,
+    AdminInventoryItem,
+    AdminLowStockItem,
+)
+
+__all__ = [
+    "InventoryCreate",
+    "InventoryAdjust",
+    "InventoryReserve",
+    "InventoryResponse",
+    "InventoryListResponse",
+    "InventoryTransactionResponse",
+    "AdminInventoryItem",
+    "AdminLowStockItem",
+]