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/features/upload/UploadPage.tsx

@@ -137,9 +137,11 @@ function TempCard({
             No preview
           </div>
         )}
-        {temp.status === "visual_match" && temp.iqdb_data?.length ? (
+        {temp.status === "visual_match" &&
+        (temp.iqdb_data?.length ?? 0) + (temp.visual_matches?.length ?? 0) > 0 ? (
           <span className="absolute left-2 top-2 rounded-full bg-ctp-peach/90 px-1.5 py-0.5 font-mono text-[10px] font-medium text-ctp-crust">
-            {temp.iqdb_data.length} similar
+            {(temp.iqdb_data?.length ?? 0) + (temp.visual_matches?.length ?? 0)}{" "}
+            similar
           </span>
         ) : null}
       </div>
@@ -282,6 +284,13 @@ function MetadataModal({
     });
   }
 
+  function discardUpload() {
+    void api(`/api/uploads/${temp.temp_id}/`, { method: "DELETE" }).then(() => {
+      void queryClient.invalidateQueries({ queryKey: ["uploads"] });
+      onClose();
+    });
+  }
+
   return (
     <div
       className="fixed inset-0 z-[70] flex items-start justify-center overflow-y-auto bg-ctp-crust/70 px-4 py-10 backdrop-blur-sm"
@@ -364,6 +373,54 @@ function MetadataModal({
 
             {tab === "link" ? (
               <div className="flex flex-col gap-3">
+                {temp.visual_matches && temp.visual_matches.length > 0 ? (
+                  <div>
+                    <p className="text-xs font-medium uppercase tracking-wide text-ctp-overlay1">
+                      Already in your library
+                    </p>
+                    <div className="mt-2 flex flex-col gap-2">
+                      {temp.visual_matches.map((match) => (
+                        <div
+                          key={match.j_id}
+                          className="flex items-center gap-3 rounded-md border border-ctp-surface0 bg-ctp-surface0/40 p-2"
+                        >
+                          <img
+                            src={`/api/files/${match.j_id}/thumbnail/`}
+                            alt={match.j_id}
+                            loading="lazy"
+                            className="h-12 w-12 shrink-0 rounded object-cover"
+                          />
+                          <span className="min-w-0 flex-1">
+                            <Link
+                              to={`/detail/${match.j_id}`}
+                              className="font-mono text-[11px] text-ctp-blue hover:underline"
+                            >
+                              {match.j_id}
+                            </Link>
+                            <span className="block truncate font-mono text-[11px] text-ctp-subtext0" title={match.filename}>
+                              {match.filename}
+                            </span>
+                          </span>
+                          <span className="shrink-0 font-mono text-[11px] text-ctp-peach">
+                            {match.similarity}%
+                          </span>
+                        </div>
+                      ))}
+                    </div>
+                    <p className="mt-2 text-[11px] text-ctp-overlay0">
+                      This upload looks like it is already in the library —
+                      you can discard it below.
+                    </p>
+                    <button
+                      type="button"
+                      onClick={discardUpload}
+                      className="mt-2 rounded-md px-1.5 py-0.5 font-mono text-[10px] text-ctp-red transition hover:bg-ctp-red/15"
+                    >
+                      discard this upload
+                    </button>
+                  </div>
+                ) : null}
+
                 {temp.iqdb_data && temp.iqdb_data.length > 0 ? (
                   <div>
                     <p className="text-xs font-medium uppercase tracking-wide text-ctp-overlay1">