Attach a passenger's frequent flyer / loyalty programme account number to a Duffel flight offer or order.

Steps

1. Choose a flow: include loyalty data at search time (Offer Request) to unlock discounted fares/extra baggage, or add it after search via the Offer Passenger update endpoint.
2. For the search-time flow, call POST /air/offer_requests and, for each passenger, include a loyalty_programme_accounts array of {airline_iata_code, account_number} objects alongside that passenger's given_name/family_name (required once loyalty data is supplied).
3. For the post-search flow, call PATCH /air/offers/{offer_id}/passengers/{offer_passenger_id} with the same loyalty_programme_accounts array (plus given_name/family_name) — repeat this call for each passenger individually, since accounts aren't shared between passengers.
4. After a PATCH update, re-fetch the offer with GET /air/offers/{offer_id} since the price may change once the loyalty discount/benefit is applied.
5. Proceed to create the order as normal via POST /air/orders — loyalty_programme_accounts data does not need to be resent at order-creation time, since it's already attached to the offer's passengers.
6. If the airline rejects the number, order creation returns an invalid_loyalty_card airline_error (422) — handle by retrying without the loyalty account or asking the traveller to confirm their membership number.