Autumn: setup_payment and payment_method expand

[RhysSullivan]

Adds the card flows to the Autumn emulator so an app can exercise "change billing card" end to end. Behaviour verified against the live Autumn sandbox API.

• POST /v1/billing.setup_payment returns a hosted setup URL (/checkout/setup/:id). Completing the form redirects to success_url; the card lands only when the session is settled (POST /checkout/setup/:id/settle or the customer-wide /checkout/settle), mirroring the webhook race. As in real Autumn, a setup session never replaces an existing default card.
• billing.open_customer_portal now has a hosted page at /checkout/portal/:customerId with a payment method form that updates the card immediately, and a return link to return_url.
• customers.get_or_create honours expand: ["payment_method"]. The key is omitted when no card is on file, as upstream does.
• Missing customer_id on setup_payment answers invalid_inputs, matching upstream.
• Version 0.14.2.

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