Returns the current lifecycle state of a payout. Identify the payout by exactly one of payout_id or idempotency_key — supplying neither or both returns 400 Bad Request. The response is lifecycle-only (PII-free); full per-resource details are fetched from the existing per-resource endpoints (GET /participants/{code}, GET /payments/external_accounts/{id}, GET /payments/{payment_id}) once the caller has the identifier. All queries are scoped to the caller's platform_code; a payout_id belonging to another tenant returns 404 Not Found (indistinguishable from a non-existent payout).