Upsert up to 1000 client accounts in a single request. Set org_group on every item if your organization uses org groups. Each item is keyed on (org_group, sub_org_group, external_id) — existing rows are updated; missing rows are inserted. Per-row savepoints: a single bad row does not roll back the rest. Always returns HTTP 200 with per-row created / updated / failed status. id in the item payload is ignored.