Create a SetupIntent to save a card without charging it

Create a Stripe SetupIntent so the frontend can collect and verify a card via Stripe Elements, without charging anything. Used identically whether someone is proactively adding a card ahead of time (still under 250 free calls) or reactively adding one after being blocked at the wall -- in either case, this just saves a payment method; the actual conversion to the paid tier happens transparently on the next API/MCP call via auth._check_monthly_limit -> usage_tracking.auto_convert_free_org_or_block, once org.default_payment_method_id is set below (via the setup_intent.succeeded webhook, not trusted directly from the client).

Returns: {"client_secret": str, "publishable_key": str} for Stripe.js/Elements on the frontend.