Upsert up to 1000 client brokers 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, client_broker_id). Per-row savepoints; HTTP 200 with per-row status.
This endpoint enforces the natural-key contract at the application layer (SELECT-then-INSERT-or-UPDATE) since the resource has no DB-level unique constraint.