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/templates/admin/partials/sidebar.html

@@ -80,9 +80,9 @@
         {{ menu_item('marketplace-products', '/admin/marketplace-products', 'database', 'Marketplace Products') }}
         {{ menu_item('vendor-products', '/admin/vendor-products', 'cube', 'Vendor Products') }}
         {{ menu_item('customers', '/admin/customers', 'user-group', 'Customers') }}
-        {# Future items - uncomment when implemented:
         {{ menu_item('inventory', '/admin/inventory', 'archive', 'Inventory') }}
         {{ menu_item('orders', '/admin/orders', 'clipboard-list', 'Orders') }}
+        {# Future items - uncomment when implemented:
         {{ menu_item('shipping', '/admin/shipping', 'truck', 'Shipping') }}
         #}
     {% endcall %}