Get Basket checkout configuration

Returns the legal and clinical pre-flight data a shop frontend needs to render the final checkout step:

• terms — the booking terms & conditions the customer must accept, assembled from site, offering, and basket-level sources.
• contraindications — clinical contraindications attached to any appointment offerings in the basket (filtered to entries the customer must positively confirm).
• voucher_payment_enabled — whether external voucher redemption is configured for the basket's site.