Provision a domain ($49/mo subscription item)

Body for POST /v1/provisioned-domains — request to provision one domain at the $49/mo price. The route attaches a SubscriptionItem to the customer's existing subscription (no Checkout redirect) and returns the resulting Stripe subscription and item ids. The local provisioned_domains row is upserted in the same request so a retried POST hits the route's pre-flight 409 instead of Stripe; the webhook then keeps the row in sync with subsequent Stripe events.

Response to a successful POST /v1/provisioned-domains.

The route attaches a new line item to the caller's existing Stripe subscription (no Checkout redirect needed — the customer already authorised the payment method for their tier plan), so the response surfaces the Stripe identifiers directly instead of a checkout URL.