Check whether a card qualifies for a discount

Checks the card the customer has just entered against the merchant's live bank discount campaigns and, when one applies, returns how much comes off and what the fee would be. This is the call a checkout makes the moment the card number is complete, so the customer sees the bank discount before pressing pay. Served from the fep.kashier.io hosts. A card with no discount still returns 200 — the absence of a response object is the signal, not an error.