Validate Form Coupon

POST 
/api/forms/public/:form_id/validate-coupon

PUBLIC endpoint — validate a promo code against a form's order context. No auth required (public forms are unauthenticated). The tenant is resolved from the form record, then the shared CouponValidationService prices the discount. Returns a CouponQuote; the charge is still re-validated server-side at submit time, so this is a UX preview, not the source of truth.

Request​