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/admin.py

@@ -0,0 +1,26 @@
+# app/modules/inventory/routes/admin.py
+"""
+Inventory module admin routes.
+
+This module wraps the existing admin inventory routes and adds
+module-based access control. Routes are re-exported from the
+original location with the module access dependency.
+"""
+
+from fastapi import APIRouter, Depends
+
+from app.api.deps import require_module_access
+
+# Import original router
+from app.api.v1.admin import inventory as inventory_routes
+
+# Create module-aware router
+admin_router = APIRouter(
+    prefix="/inventory",
+    dependencies=[Depends(require_module_access("inventory"))],
+)
+
+# Re-export all routes from the original module with module access control
+# The routes are copied to maintain the same API structure
+for route in inventory_routes.router.routes:
+    admin_router.routes.append(route)