Create a hosted Stripe checkout for credits

Creates a payment link, not a charge. Requires write permission and an Idempotency-Key. Uses only the credential workspace; team purchases require the active owner. Amount is USD before checkout taxes and discounts. No payment data is accepted. Reuse identical inputs and the same key after an uncertain result. Successful responses replay for 24 hours; the checkout URL can expire sooner. Only a verified successful payment webhook grants credits.