Create Credit Pack Checkout

One-time Stripe Checkout to buy a prepaid credit pack (PAYG tier).

Unlike the subscription checkout, this is mode="payment" — a single charge of credits × $0.15 with no recurring plan. The card is collected here (a PAYG org needs none until its first purchase). Multiple concurrent purchases are legitimate, so this does NOT use the one-subscription-per-org single-flight guard; a Stripe idempotency_key guards double-clicks and the webhook credits the ledger idempotently via a UNIQUE stripe_ref.