Invoke Billing Topup

Off-session card charge → credits. Idempotency is at the STRIPE level: the client's Idempotency-Key is threaded into charge_for_credits (Stripe dedups the charge, the PaymentIntent id dedups the grant), matching the legacy /billing/topup exactly. On 3DS/SCA the op raises a 402 carrying the PaymentIntent client_secret, which we surface at the TOP LEVEL of the problem body — problem.client_secret — because that's what the topup page reads to drive confirmCardPayment.