Add credits by paying this request (Machine Payments Protocol, HTTP 402)

The first call returns 402 with a payment challenge (WWW-Authenticate: Payment). An MPP client pays it — card through a Stripe Link agent wallet, or USDC on Tempo — and retries with Authorization: Payment <credential>. Because Authorization carries the payment, send the Varosity key as X-API-Key. amountUsd: whole dollars 5–500 (default 10); credits equal dollars paid; each payment is credited once, to the account that requested it.