Payout Callback

Payout outcome from integration-service (webhook forward or reconciler). Body: {payout_reference?, order_key?, status: paid|failed|submit_not_received, amount?, currency?, failure_reason?, gateway_payload?} Idempotent: Redis NX dedup + FOR UPDATE lock + terminal-state guards. submit_not_received uses a distinct dedup key so it doesn't collide with a future paid/failed callback for the same reference.