Body is an ARRAY of custom data definitions. Validation is all-or-nothing: if any item is invalid, nothing is created and the response only contains the invalid items ({custom_data, success: false, errors}). Otherwise each item is created and returns {success: true, custom_data_key, id} (or errors: ["Creation failed"]). A body that is not a non-empty array of objects returns [{success: false, custom_data: "", errors: ["CD0002 - Invalid user data"]}]. Business errors are returned with HTTP 200 and success: false; each message is prefixed with its code (CDxxxx - ...). Emits a CustomDataCreatedEvent per created item.