Relink one face to another person on the selected images only (super-admin).

The bulk twin of /assignFace's single_existing: on each selected image, the face boxes linked to the looked-up person are pointed at the target person. Nothing else moves — the source person keeps its name and every face on images that were not selected — which is the point: a person wrongly matched across some photos is fixed without renaming (which would change every photo) or merging (which would move all of them).