feat: extract inventory, orders, and marketplace modules (Phase 4)

Extract three additional modules following the billing module pattern:

Inventory Module (app/modules/inventory/):
- Stock management and tracking
- Inventory locations
- Low stock alerts
- Admin and vendor routes with module access control

Orders Module (app/modules/orders/):
- Order management and fulfillment
- Order item exceptions
- Bulk operations and export
- Admin and vendor routes with module access control

Marketplace Module (app/modules/marketplace/):
- Letzshop integration
- Product sync
- Marketplace import
- Depends on inventory module
- Admin and vendor routes with module access control

Admin router updated:
- Uses module routers with require_module_access dependency
- Legacy router includes commented out
- Routes verified: 15 inventory, 16 orders, 42 marketplace

All 31 module tests passing.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/orders/__init__.py

@@ -0,0 +1,23 @@
+# app/modules/orders/__init__.py
+"""
+Orders Module - Order processing and fulfillment.
+
+This module provides:
+- Order management and tracking
+- Order fulfillment workflow
+- Order item exceptions handling
+- Bulk order operations
+- Order export and reporting
+
+Routes:
+- Admin: /api/v1/admin/orders/*, /api/v1/admin/order-item-exceptions/*
+- Vendor: /api/v1/vendor/orders/*, /api/v1/vendor/order-item-exceptions/*
+
+Menu Items:
+- Admin: orders
+- Vendor: orders
+"""
+
+from app.modules.orders.definition import orders_module
+
+__all__ = ["orders_module"]