Create Signup Grant Setup Intent
Billing
Create Signup Grant Setup Intent
Open a Stripe SetupIntent so the user can verify a card and claim the $20 signup grant.
Stripe runs Radar / CVC / AVS / 3DS checks during SetupIntent confirmation;
on success Stripe fires setup_intent.succeeded which the webhook handler
turns into an atomic claim_signup_grant call + Croesus credit
(idempotency_key signup:<user_id>, matching the retired auth-hook path
so double-claims via either path are impossible).
POST
Create Signup Grant Setup Intent