Upload board: dismiss all, real previews for indexed records

- 'dismiss all' clears every indexed record at once
- Indexed cards show the actual file preview: completed records now get a
  signed library media URL (raw for images, thumbnail for videos) so
  <img>/<video> tags can load it, including items hidden from guests
- Media raw/thumbnail endpoints accept the signature for anonymous
  requests and fall back to the normal guest-filtered path otherwise
- Guest blacklist keeps a persistent Redis mirror: an expired TTL or an
  unreachable e621 keeps the last successful list instead of falling
  back to the small local list

frontend/src/lib/types.ts

@@ -135,6 +135,7 @@ export interface TempUpload {
   iqdb_data: E621IqdbCandidate[] | null;
   library_j_id: string | null;
   file_url: string | null;
+  preview_url: string | null;
   created_at: string;
   updated_at: string;
 }