Duplicates, delete & storage, users page with J-ID avatars

Backend:
- Perceptual hashes (aHash/dHash/pHash/wHash via imagehash, no imgdd)
  stored on items, computed on upload/download and by the new
  compute_visual_hashes command
- Duplicates API: exact duplicates (multi-location items), visual matches
  for one item, union-find similarity groups with pagination
- Delete API with ownership/staff checks, per-item and per-copy deletion,
  watched-folder path validation; storage overview and temp cleanup;
  file list accepts j_ids batches
- Staged uploads are flagged visual_match with their library matches
  (threshold via VISUAL_MATCH_THRESHOLD)
- Staff users API: list with upload counts, set role and avatar by J-ID;
  User.avatar FK with signed avatar URLs
- Download threads close their DB connection and stale tasks are reaped,
  keeping behaviour Gunicorn-friendly

Frontend:
- /duplicates: exact duplicate groups with per-copy delete, visual
  similarity controls, search similar to a J-ID, paginated groups with
  selection, bulk delete and dismiss
- /delete: storage cards, delete by J-ID with preview grid, temp cleanup
- /users: staff directory with role selects and avatar J-ID inputs
- Nav + command palette entries; top-bar avatar; upload cards and the
  metadata modal show library visual matches

backend/apps/library/views.py

@@ -16,7 +16,7 @@ from rest_framework.response import Response
 from rest_framework.views import APIView
 
 from . import services
-from .downloads import start_download_task
+from .downloads import reap_stale_downloads, start_download_task
 from .models import DownloadTask, MediaItem
 from .permissions import CanUpload, IsUploaderOrStaffOrReadOnly
 from .serializers import DownloadTaskSerializer, MediaItemSerializer
@@ -41,6 +41,15 @@ class MediaItemViewSet(
         )
         if not self.request.user.is_authenticated:
             queryset = queryset.filter(hidden_from_guests=False)
+        j_ids = self.request.query_params.get("j_ids", "").strip()
+        if j_ids:
+            numeric_ids = []
+            for value in j_ids.split(","):
+                text = value.strip()
+                number = text[2:] if text.upper().startswith("J-") else text
+                if number.isdigit():
+                    numeric_ids.append(int(number))
+            queryset = queryset.filter(pk__in=numeric_ids)
         search = self.request.query_params.get("search", "").strip()
         if search:
             queryset = queryset.filter(locations__rel_path__icontains=search)
@@ -203,6 +212,7 @@ class DownloadTaskViewSet(
     http_method_names = ["get", "post", "head", "options"]
 
     def get_queryset(self):
+        reap_stale_downloads()
         queryset = DownloadTask.objects.select_related("library_item")
         user = self.request.user
         if not (user.is_staff or user.is_superuser):