List Access Grants

List access-grant rows. Mode selected by query params:

• client_id only ⇒ all rows for the client across every dataset (client-side "Block Restrictions" section, read-only).
• client_id + dataset_id ⇒ all cuts' rows for that client+dataset (shared editor).
• dataset_id + scope=dataset ⇒ all rows for the dataset across every cut, with client info (dataset-side "Manage access" list).
• dataset_id + cut ⇒ rows for that exact (dataset, cut), with client info.

cut is ignored when scope=dataset or client_id is set.