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/inventory/routes/__init__.py

@@ -0,0 +1,12 @@
+# app/modules/inventory/routes/__init__.py
+"""
+Inventory module route registration.
+
+This module provides functions to register inventory routes
+with module-based access control.
+"""
+
+from app.modules.inventory.routes.admin import admin_router
+from app.modules.inventory.routes.vendor import vendor_router
+
+__all__ = ["admin_router", "vendor_router"]