feat: add admin inventory management (Phase 1)

- Add admin API endpoints for inventory management
- Add inventory page with vendor selector and filtering
- Add admin schemas for cross-vendor inventory operations
- Support digital products with unlimited inventory
- Add integration tests for admin inventory API
- Add inventory management guide documentation

Mirrors vendor inventory functionality with admin-level access.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/api/v1/admin/__init__.py

@@ -32,11 +32,13 @@ from . import (
     companies,
     content_pages,
     dashboard,
+    inventory,
     letzshop,
     logs,
     marketplace,
     monitoring,
     notifications,
+    orders,
     products,
     settings,
     tests,
@@ -98,7 +100,7 @@ router.include_router(dashboard.router, tags=["admin-dashboard"])
 
 
 # ============================================================================
-# Product Catalog
+# Vendor Operations (Product Catalog, Inventory & Orders)
 # ============================================================================
 
 # Include marketplace product catalog management endpoints
@@ -107,6 +109,12 @@ router.include_router(products.router, tags=["admin-marketplace-products"])
 # Include vendor product catalog management endpoints
 router.include_router(vendor_products.router, tags=["admin-vendor-products"])
 
+# Include inventory management endpoints
+router.include_router(inventory.router, tags=["admin-inventory"])
+
+# Include order management endpoints
+router.include_router(orders.router, tags=["admin-orders"])
+
 
 # ============================================================================
 # Marketplace & Imports