Hold Flight

Arm a 24h fare hold on the trip's pinned flight (FAC-940).

Basket lock (ENG-21867 follow-on): held_items are the bag/seat lines locked with the fare. The gateway forwards them verbatim — the brain owns every semantic validator (identity, amounts, all-or-nothing), so this model stays deliberately permissive: a stricter mirror here would just be a second copy that drifts. Without this field the default extra=ignore silently stripped the lines and every hold armed fare-only.