Re-sign visual-match thumbnails on every detail fetch

Match rows stored a signed URL minted when the scan ran, so it aged out (or
used the pre-stable signing scheme) and the modal showed broken tiles even
after legacy signatures were fixed. Rows now carry item_id/j_id and the
detail serializer mints a fresh thumbnail URL per request; matches whose
item no longer exists are dropped.

backend/apps/library/uploads.py

@@ -60,6 +60,7 @@ def match_hashes(hashes, index, limit=10, user=None, request=None):
         location = item.locations.first()
         matches.append(
             {
+                "item_id": item.id,
                 "j_id": f"J-{item.id}",
                 "filename": Path(location.rel_path).name if location else item.md5,
                 "similarity": round(similarity * 100, 1),