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/views.py

@@ -3,6 +3,7 @@ from pathlib import Path
 from urllib.parse import urlparse
 
 from django.conf import settings
+from django.core import signing
 from django.db.models import Min
 from django.http import Http404
 from django.shortcuts import get_object_or_404
@@ -67,9 +68,32 @@ class MediaItemViewSet(
         self.check_object_permissions(self.request, obj)
         return obj
 
+    def _signed_media_item(self, request, action_name):
+        """Signed media URLs let <img>/<video> tags bypass guest filtering."""
+        if request.user.is_authenticated:
+            return None
+        signature = request.query_params.get("sig")
+        if not signature:
+            return None
+        try:
+            payload = signing.loads(
+                signature, salt=services.MEDIA_FILE_SALT, max_age=86400
+            )
+        except signing.BadSignature:
+            return None
+        if payload.get("action") != action_name:
+            return None
+        return MediaItem.objects.filter(pk=payload.get("item")).first()
+
+    def _media_object(self, request, action_name):
+        item = self._signed_media_item(request, action_name)
+        if item is not None:
+            return item
+        return self.get_object()
+
     @action(detail=True, methods=["get"])
     def raw(self, request, pk=None):
-        item = self.get_object()
+        item = self._media_object(request, "raw")
         location = item.locations.first()
         if location is None:
             return Response(
@@ -82,7 +106,7 @@ class MediaItemViewSet(
 
     @action(detail=True, methods=["get"])
     def thumbnail(self, request, pk=None):
-        item = self.get_object()
+        item = self._media_object(request, "thumbnail")
         location = item.locations.first()
         if location is None:
             return Response(