Payload for a line-item level discount.

value is a percentage for percent or an absolute amount for
amount. A price override goes through
PUT /orders/{order_id}/items/{item_id}/price instead — a custom price is
a price, not a discount (migration 042). Percentages are bounded at 100 in
the validator below — the property's own settings schema bounds
max_percent the same way, and a 500% discount is an input error rather than
a comp.