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>

src/lib/files.ts

@@ -0,0 +1,20 @@
+/**
+ * Public base URL for objects in the (public-read) user-uploads MinIO bucket.
+ * Must be browser-reachable (MinIO is published on :9000). Configure per deployment
+ * via NEXT_PUBLIC_MINIO_URL; falls back to localhost for local dev.
+ */
+export const MINIO_PUBLIC_URL = (
+  process.env.NEXT_PUBLIC_MINIO_URL ?? "http://localhost:9000"
+).replace(/\/$/, "");
+
+/** Build a public object URL from a file's bucket + key. */
+export function publicFileUrl(bucket: string, key: string): string {
+  return `${MINIO_PUBLIC_URL}/${bucket}/${key}`;
+}
+
+export interface UploadedFile {
+  id: string;
+  name: string;
+  url: string;
+  mime_type?: string;
+}