Stripe hosted checkout for a clinic plan (§2.3).

Refusals, and what each one means to the owner: 403 she is not the owner; 409 talk_to_us her clinic is not in Australia, where self-serve does not run and the screen offers a conversation instead; 409 plan_already_set her clinic already has a plan, which is what stops a second subscription being opened on a clinic that is already paying - unless Stripe has ended the subscription, when a new checkout is the one way back (_repurchase_allowed); 409 plan_downgrade a Pro clinic coming back as Lite, which activate_clinic_plan would refuse from inside the webhook, after Stripe had taken the money (#9185).