Start a credit top-up checkout

Returns an embedded Stripe Checkout `clientSecret` for a pay-as-you-go credit top-up (amount entered by the customer in Checkout). The wallet is credited when the resulting invoice is paid. The customer may optionally save the card for future use; the first saved card becomes the default payment method automatically. Rejected with `409` while a subscription is active — subscription accounts don't draw on the wallet. Not available during impersonation.