Approve Face Match Suggestion Endpoint

Approve a face match suggestion: merge candidate person into reference person (reassign all image_faces from candidate to reference, delete candidate person and their Rekognition face). Optional chosen_name: set the reference person's name to this after merge (e.g. user-selected name).

Process: (1) DB transaction: reassign image_faces, delete suggestion and candidate person, update name/folder. (2) Rekognition: delete candidate face from collection. (3) Emits (person.folders, persons.updated, face_match_suggestions.updated) run in a background task so the response returns immediately; previously get_project_persons (which built presigned URLs for every person) ran inline and could take many seconds on large projects.