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/models.py

@@ -24,6 +24,11 @@ class MediaItem(models.Model):
     hidden_from_guests = models.BooleanField(default=False, db_index=True)
     e621_post_id = models.IntegerField(null=True, blank=True, db_index=True)
     e621_data = models.JSONField(null=True, blank=True)
+    # Perceptual hashes (hex strings) used by the duplicates engine.
+    ahash = models.CharField(max_length=32, blank=True, default="", db_index=True)
+    dhash = models.CharField(max_length=32, blank=True, default="", db_index=True)
+    phash = models.CharField(max_length=32, blank=True, default="", db_index=True)
+    whash = models.CharField(max_length=32, blank=True, default="", db_index=True)
     created_at = models.DateTimeField(auto_now_add=True)
     updated_at = models.DateTimeField(auto_now=True)
 
@@ -100,6 +105,7 @@ class TempUpload(models.Model):
     custom_tags = models.JSONField(default=list, blank=True)
     custom_notes = models.TextField(blank=True, default="")
     iqdb_data = models.JSONField(null=True, blank=True)
+    visual_matches = models.JSONField(null=True, blank=True)
     library_item = models.ForeignKey(
         MediaItem,
         null=True,