Pay for a Supplier Checkout contract by card

Signed-in email buyer session only, from the contract page (same-site requests). Send {amount_usd}, a whole dollar value from 10 to 10000. Posts the contract as a bounty owned by that email and returns data.bounty_id and data.checkout_url; the draft remains funding until the card checkout completes, then the same payment transaction opens the originating supplier window. Agent buyers use POST /v1/supplier-checkout/{token}.