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/config/settings.py

@@ -166,6 +166,9 @@ GUEST_BLACKLIST_FALLBACK = [
 ]
 GUEST_BLACKLIST_TTL = int(os.getenv("GUEST_BLACKLIST_TTL", "3600"))
 
+# Similarity threshold for flagging staged uploads that match library items.
+VISUAL_MATCH_THRESHOLD = float(os.getenv("VISUAL_MATCH_THRESHOLD", "0.9"))
+
 # Redis cache (run via docker compose at the repo root), shared by web
 # workers and management commands (e.g. the mirrored guest blacklist).
 CACHES = {