feat(admin): media library + upload component (replace URL fields)

- /admin/files Media Library: drag-drop multi-upload, thumbnails, copy-URL, delete
- FileUploadField replaces raw URL inputs; new "image" field type in AdminResource;
  wired into category image
- upload proxy /api/admin/files/upload: browser → Next → presigned PUT (server-side,
  reaches minio:9000) → confirm → returns public URL
- user-uploads bucket is public-read; public base via NEXT_PUBLIC_MINIO_URL

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.v2.yml

@@ -276,6 +276,7 @@ services:
         # V2 gateway: browser-facing base (host port) baked in at build time.
         NEXT_PUBLIC_API_URL:      "${NEXT_PUBLIC_API_URL:-http://localhost:${GATEWAY_PORT:-8080}/v1}"
         NEXT_PUBLIC_TENANT_SLUG:  "${NEXT_PUBLIC_TENANT_SLUG:-flatrender}"
+        NEXT_PUBLIC_MINIO_URL:    "${NEXT_PUBLIC_MINIO_URL:-http://localhost:9000}"
     container_name: fr2-frontend
     restart: unless-stopped
     ports: