Create a product alert for a contact
Register a contact to receive an alert for a specific product event, such as `back_in_stock`. At least one contact identifier (`ext_id`, `email`, or `sms`) must be provided; when multiple are given, priority is `ext_id` > `email` > `sms`. Returns a `404` error if the product ID does not exist, and a `403` error if product alerts are not enabled for the account.