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/serializers.py

@@ -150,6 +150,7 @@ class TempUploadSerializer(serializers.ModelSerializer):
     iqdb_checked = serializers.SerializerMethodField()
     processing = serializers.SerializerMethodField()
     similar_count = serializers.SerializerMethodField()
+    visual_matches = serializers.SerializerMethodField()
 
     class Meta:
         model = TempUpload
@@ -242,6 +243,57 @@ class TempUploadSerializer(serializers.ModelSerializer):
     def get_similar_count(self, obj):
         return len(obj.iqdb_data or []) + len(obj.visual_matches or [])
 
+    @staticmethod
+    def _visual_item_id(entry):
+        if not isinstance(entry, dict):
+            return None
+        item_id = entry.get("item_id")
+        if item_id is None:
+            j_id = str(entry.get("j_id") or "")
+            if j_id.upper().startswith("J-"):
+                j_id = j_id[2:]
+            item_id = j_id if j_id.isdigit() else None
+        try:
+            return int(item_id)
+        except (TypeError, ValueError):
+            return None
+
+    def get_visual_matches(self, obj):
+        """Rebuild match rows with fresh signed thumbnail URLs.
+
+        Storing the signed URL meant it aged out (or came from an older
+        signing scheme) and the "Already in your library" grid showed broken
+        tiles. The stored rows only carry the item reference now.
+        """
+        entries = obj.visual_matches or []
+        if not entries:
+            return entries
+        wanted = {}
+        for entry in entries:
+            item_id = self._visual_item_id(entry)
+            if item_id is not None:
+                wanted[item_id] = None
+        items = MediaItem.objects.in_bulk(list(wanted))
+        user = self._request_user()
+        request = self.context.get("request")
+        matches = []
+        for entry in entries:
+            item_id = self._visual_item_id(entry)
+            item = items.get(item_id) if item_id is not None else None
+            if item is None:
+                continue
+            matches.append(
+                {
+                    "j_id": f"J-{item.id}",
+                    "filename": entry.get("filename") or item.md5,
+                    "similarity": entry.get("similarity"),
+                    "thumbnail_url": signed_media_url(
+                        item, user, "thumbnail", request=request
+                    ),
+                }
+            )
+        return matches
+
 
 class TempUploadListSerializer(TempUploadSerializer):
     """Compact staged-upload row for the board and the status polling.