Start a subscription

Creates a Stripe Checkout session and returns its URL. Open that URL in a browser or in-app browser tab; Stripe takes the payment details, and the subscription is reconciled by webhook when checkout completes. Do not collect card details yourself.

For new subscribers only. An organisation that already has a live subscription gets 409 and should be sent to POST /api/v1/billing/change-plan or the customer portal instead.

Broker admins only, and refuses API-token auth.