Saved-card status for the billing page.

Reports the exact card the auto-recharge sweep would charge (the shared _pick_payment_method pick: default PM, else the newest saved card) so the UI can warn when auto-recharge has nothing workable to bill. Read only: never creates a Stripe customer, and the Stripe lookups run in a worker thread so a slow Stripe API cannot stall the event loop on every /billing page load.

Shapes: {hasCard: true, brand, last4, expMonth, expYear, expired} or {hasCard: false, reason: not_configured|no_customer|no_card|stripe_error}. The UI treats stripe_error/not_configured as unknown, not as missing.