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>

static/admin/js/init-alpine.js

@@ -67,7 +67,9 @@ function data() {
     'marketplace-products': 'vendorOps',
     'vendor-products': 'vendorOps',
     customers: 'vendorOps',
-    // Future: inventory, orders, shipping will map to 'vendorOps'
+    inventory: 'vendorOps',
+    orders: 'vendorOps',
+    // Future: shipping will map to 'vendorOps'
     // Marketplace
     'marketplace-letzshop': 'marketplace',
     // Content Management