Fix hidden library items not rendering in the browser

Items flagged hidden_from_guests (blacklisted tags) returned 404 for
<img> requests since tags cannot send the auth header. The API now
exposes signed raw_url/thumbnail_url fields (mirroring upload previews
and avatars), and the SPA uses them in the gallery, detail view,
duplicates and delete screens, and upload visual matches.

frontend/src/features/upload/UploadPage.tsx

@@ -385,7 +385,10 @@ function MetadataModal({
                           className="flex items-center gap-3 rounded-md border border-ctp-surface0 bg-ctp-surface0/40 p-2"
                         >
                           <img
-                            src={`/api/files/${match.j_id}/thumbnail/`}
+                            src={
+                              match.thumbnail_url ??
+                              `/api/files/${match.j_id}/thumbnail/`
+                            }
                             alt={match.j_id}
                             loading="lazy"
                             className="h-12 w-12 shrink-0 rounded object-cover"