Create Auto Top Up Setup Intent

Open a SetupIntent so the org can save a card for off-session auto top-ups.

Croesus owns the org’s Stripe billing identity: it resolves (or creates) the customer and opens the SetupIntent. This endpoint just proxies the client secret to the dashboard. On confirmation Stripe fires setup_intent.succeeded which the webhook turns into a Croesus billing-profile update (the saved card).