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/accounts/models.py

@@ -15,6 +15,13 @@ class User(AbstractUser):
     ]
 
     role = models.CharField(max_length=20, choices=ROLE_CHOICES, default=ROLE_USER)
+    avatar = models.ForeignKey(
+        "library.MediaItem",
+        null=True,
+        blank=True,
+        on_delete=models.SET_NULL,
+        related_name="+",
+    )
     e621_username = models.CharField(max_length=100, blank=True, default="")
     e621_api_key = models.CharField(max_length=100, blank=True, default="")
     e621_base_url = models.CharField(max_length=200, default="https://e621.net")