Create a subscription ON the saved default card (one-click). payment_behavior=default_incomplete leaves the first
invoice unpaid with a PaymentIntent the browser confirms (confirmCardPayment → 3DS in-page) → the subscription
activates → invoice.paid grants the cycle's credits. Returns the first invoice's client secret + the subscription id,
or null when there's no default card.
Create a subscription ON the saved default card (one-click). payment_behavior=default_incomplete leaves the first invoice unpaid with a PaymentIntent the browser confirms (confirmCardPayment → 3DS in-page) → the subscription activates → invoice.paid grants the cycle's credits. Returns the first invoice's client secret + the subscription id, or null when there's no default card.