Body is an ARRAY of partial updates identified by key (+ site). Validation is all-or-nothing: if any item is invalid, nothing is updated and the response only contains the invalid items ({custom_data, success: false, errors}). Otherwise each item returns {success: true, custom_data_key, message: "Update successful"} or an error entry. Renamed predefined values are propagated to linked products. Business errors are returned with HTTP 200 and success: false; each message is prefixed with its code (CDxxxx - ...). Emits a CustomDataUpdatedEvent per updated item.