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

backend/apps/library/services.py

@@ -25,6 +25,7 @@ ALLOWED_EXTENSIONS = {
 }
 VIDEO_EXTENSIONS = {".mp4", ".webm"}
 UPLOAD_FILE_SALT = "j621.upload-file"
+MEDIA_FILE_SALT = "j621.media-file"
 CHUNK_SIZE = 1024 * 1024
 RANGE_RE = re.compile(r"bytes=(\d*)-(\d*)$")