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/downloads.py

@@ -3,10 +3,12 @@
 import logging
 import threading
 import time
+from datetime import timedelta
 from pathlib import Path
 from urllib.parse import urlparse
 
 from django.conf import settings
+from django.db import connection
 from django.utils import timezone
 
 from . import services
@@ -14,6 +16,27 @@ from .models import DownloadTask
 
 logger = logging.getLogger(__name__)
 
+STALE_AFTER = timedelta(minutes=30)
+
+
+def reap_stale_downloads():
+    """Mark tasks left hanging by a recycled worker as failed.
+
+    Gunicorn recycles workers (--max-requests, timeouts); a download thread
+    dies with its worker, so long-stuck tasks are surfaced as errors instead
+    of pretending to run forever.
+    """
+    cutoff = timezone.now() - STALE_AFTER
+    return DownloadTask.objects.filter(
+        status__in=[DownloadTask.STATUS_PENDING, DownloadTask.STATUS_DOWNLOADING],
+        updated_at__lt=cutoff,
+    ).update(
+        status=DownloadTask.STATUS_ERROR,
+        error="The worker restarted before this download finished.",
+        speed=None,
+        updated_at=timezone.now(),
+    )
+
 
 def start_download_task(task_id):
     thread = threading.Thread(target=run_download_task, args=(task_id,), daemon=True)
@@ -79,6 +102,7 @@ def run_download_task(task_id):
         )
         item, _, location, _ = services.index_file(destination, folder)
         services.rename_location_to_j_id(item, location)
+        services.ensure_visual_hashes(item)
 
         update_fields = []
         if item.uploaded_by_id is None and task.user_id is not None:
@@ -120,3 +144,7 @@ def run_download_task(task_id):
             speed=None,
             updated_at=timezone.now(),
         )
+    finally:
+        # Background threads hold their own DB connection; release it so
+        # Gunicorn workers do not leak connections when threads finish.
+        connection.close()