Order and get a payment link

Creates an unpaid order and returns the provider's payment link (checkout_url) for the customer to open and pay themselves; nothing is charged by this call. Show the price (POST /quotes) and get the customer's confirmation first. rotating_residential takes a plan_id and is one-time only (subscriptions are started in the dashboard); static_isp takes allocations [{ plan_id, quantity }] and resource_days (30 or 90). Poll GET /orders/{orderId} for payment and delivery. The Idempotency-Key header makes a retry return the same order and link. 422 checkout_unavailable when ordering is switched off.