Push Person To Brand Endpoint

Migration helper: convert a mislabeled PERSON (e.g. "Mercedes" wrongly tagged as a person) into an org-global BRAND. For every image where the person appears, adds a brand tag with the same name (so the images keep the association, just re-categorized person -> brand), registers (or merges into) the org brand of that name (union of project memberships), then deletes the person (cascading its face boxes + Rekognition entry). Body: {personId}.