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/marketplace/__init__.py

@@ -0,0 +1,25 @@
+# app/modules/marketplace/__init__.py
+"""
+Marketplace Module - Letzshop integration.
+
+This module provides:
+- Letzshop product sync
+- Marketplace import operations
+- Product catalog synchronization
+- Order import from marketplace
+
+Dependencies:
+- Requires: inventory module (for product management)
+
+Routes:
+- Admin: /api/v1/admin/marketplace/*, /api/v1/admin/letzshop/*
+- Vendor: /api/v1/vendor/marketplace/*, /api/v1/vendor/letzshop/*
+
+Menu Items:
+- Admin: marketplace-letzshop
+- Vendor: marketplace, letzshop
+"""
+
+from app.modules.marketplace.definition import marketplace_module
+
+__all__ = ["marketplace_module"]