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/definition.py

@@ -0,0 +1,70 @@
+# app/modules/inventory/definition.py
+"""
+Inventory module definition.
+
+Defines the inventory module including its features, menu items,
+and route configurations.
+"""
+
+from app.modules.base import ModuleDefinition
+from models.database.admin_menu_config import FrontendType
+
+
+def _get_admin_router():
+    """Lazy import of admin router to avoid circular imports."""
+    from app.modules.inventory.routes.admin import admin_router
+
+    return admin_router
+
+
+def _get_vendor_router():
+    """Lazy import of vendor router to avoid circular imports."""
+    from app.modules.inventory.routes.vendor import vendor_router
+
+    return vendor_router
+
+
+# Inventory module definition
+inventory_module = ModuleDefinition(
+    code="inventory",
+    name="Inventory Management",
+    description=(
+        "Stock level tracking, inventory locations, low stock alerts, "
+        "and product catalog management."
+    ),
+    features=[
+        "inventory_basic",  # Basic stock tracking
+        "inventory_locations",  # Multiple warehouse locations
+        "low_stock_alerts",  # Automated low stock notifications
+        "inventory_purchase_orders",  # Purchase order management
+        "product_management",  # Product catalog CRUD
+        "inventory_transactions",  # Stock movement history
+        "inventory_import",  # Bulk inventory import
+    ],
+    menu_items={
+        FrontendType.ADMIN: [
+            "inventory",  # Platform-wide inventory view
+            "vendor-products",  # Product catalog management
+        ],
+        FrontendType.VENDOR: [
+            "products",  # Vendor product catalog
+            "inventory",  # Vendor inventory management
+        ],
+    },
+    is_core=False,
+)
+
+
+def get_inventory_module_with_routers() -> ModuleDefinition:
+    """
+    Get inventory module with routers attached.
+
+    This function attaches the routers lazily to avoid circular imports
+    during module initialization.
+    """
+    inventory_module.admin_router = _get_admin_router()
+    inventory_module.vendor_router = _get_vendor_router()
+    return inventory_module
+
+
+__all__ = ["inventory_module", "get_inventory_module_with_routers"]