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

frontend/src/components/CommandPalette.tsx

@@ -1,4 +1,5 @@
 import {
+  Copy,
   FolderOpen,
   Globe,
   LogIn,
@@ -6,7 +7,9 @@ import {
   Menu,
   Search,
   Settings,
+  Trash2,
   Upload,
+  Users,
   type LucideIcon,
 } from "lucide-react";
 import { useMemo, useState } from "react";
@@ -59,6 +62,18 @@ function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
               icon: Upload,
               run: () => navigate("/upload"),
             },
+            {
+              id: "duplicates",
+              label: "Find duplicates",
+              icon: Copy,
+              run: () => navigate("/duplicates"),
+            },
+            {
+              id: "delete",
+              label: "Delete files & storage",
+              icon: Trash2,
+              run: () => navigate("/delete"),
+            },
           ]
         : []),
       {
@@ -94,6 +109,14 @@ function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
         icon: Settings,
         run: () => navigate("/account"),
       });
+      if (user.is_staff || user.is_superuser || user.role === "staff") {
+        list.push({
+          id: "users",
+          label: "Manage users",
+          icon: Users,
+          run: () => navigate("/users"),
+        });
+      }
       list.push({
         id: "logout",
         label: `Log out (${user.username})`,