Stripe: saved payment methods and subscriptions (feature request)

Request

Two Stripe areas that subscription and usage-billing integrations call are missing from the emulator (checked on main at 4c0422e):

1. Saved payment methods. GET /v1/payment_methods always returns data: [] (routes/payment-methods.ts), and the seed config cannot attach a card to a customer. Off-session charges (POST /v1/payment_intents with customer, payment_method, off_session=true, confirm=true) therefore cannot be exercised. Today POST /v1/payment_intents also ignores confirm=true and off_session, so the intent stays requires_confirmation. Expected: seedable pm_ cards on a customer (with customer.invoice_settings.default_payment_method), and confirm=true confirming at creation as Stripe does.
2. Subscriptions. No /v1/subscriptions routes (retrieve, update with items[0][id] + items[0][price]), no customer.subscription.created/updated/deleted events, and a mode=subscription Checkout Session completes without creating a subscription or setting session.subscription. Recurring prices (recurring: { interval }) cannot be seeded either. /v1/invoices (list) and /v1/billing_portal/sessions (create, returning a URL) would complete the picture.

Happy to help test a first cut against a real integration.