feat: add WebP migration UI and API endpoint

- Add /api/admin/migrate-images endpoint to convert JPEG/PNG to WebP
- Add WebPMigration component in admin gallery showing stats
- Upload endpoint already converts new images to WebP
- Migration saves ~30-50% storage space per image

package.json

@@ -29,6 +29,7 @@
     "@types/react-dom": "^18.3.6",
     "@vitejs/plugin-react": "^6.0.3",
     "jsdom": "^29.1.1",
+    "tsx": "^4.22.4",
     "typescript": "^5.4.5",
     "vitest": "^4.1.9"
   }