Start a Stripe Checkout session for a plan
`starter` (Free) without a tester code returns `400 BILLING_PLAN_NOT_PURCHASABLE`; start Free with `POST /billing/free/start`. A tester code's own plan always wins.
POST
/billing/checkout/session Headers
Authorizationstring RequiredBearer <session token>: the signed-in user’s Scaffold session token.
x-org-idstring Required
Body
planstringOne of
starterprofessionalteamseatsintegerTeam plan only; every other plan is always 1 seat.
successPathstringDefault
/reviewcancelPathstringDefault
/onboardingtesterAccessCodestring