Check bulk add progress
Poll the progress of a bulk member add started by 'Add members in bulk'. Returns the current status and how many donor IDs have been processed. When status is 'completed', includes a `results` object with the final breakdown: `added_ids` (successfully added), `not_found_ids` (IDs not in the DonorAtlas database), and `failed_ids` (IDs that encountered a server error — safe to retry).