Promo: addon option discounts the parent service
When the customer picks this option, the parent service price drops by X% (up to 100% = free).
Configure on each addon option: a "parent-service discount" percent (0-100). When the customer picks the option, the linked service price is reduced by that percent. 100 = the service becomes free. Mirrors the public booking summary as a green discount line with the option name. Set under /manager/addons → option edit → Advanced settings → "Kedvezmény a szolgáltatásra". The highest percent applies if multiple discounting options are chosen for one service. Works in the public flow, the embed widget, and the admin calendar booking modal alike. Replaces the earlier Tomamed-only hardcoded "Talpbetét → free Járásanalízis" rule with a generic, self-serve mechanism.