SubscriptionConfiguration

This flag tells us when we are "updating" an active subscription instead of making a new one.

Promo code applied at checkout, snapshotted so the Moloni sale document reflects exactly what was charged even if the promocode is later edited/expired. Null when no promo was applied.

Percentage discount from the applied promo code, snapshotted at checkout. Folded into the Moloni document line discounts so the document total matches the charged (discounted) payment amount.