Endpoint for adding a person to the face recognition system. If scanExistingFaces is "true", starts a background scan for matches in existing stored faces. If bulk is "true", skips folder update, persons emit, and duplicate scan; call completeBulkFaceUpload when done.

Args: name: The person's name projectId: The project ID this person belongs to category: Category/type of person (default: "person") position: The person's position/role (optional) link: Link associated with the person (optional) faceImageId: Face image ID to associate with the person (used to fetch image from S3) scanExistingFaces: If "true", scan existing images for this person after registration bulk: If "true", do not run background tasks (folders, persons emit, duplicate scan); caller must call completeBulkFaceUpload at the end

Returns: JSON response with success status and person details