UnsubscribePrivacy
Same ok for subscribed or unknown emails
A valid email always returns ok. Kumir does not reveal whether the address was on the list.
Kumir never invents a successful mailing confirmation that reveals subscribe state. POST /api/v1/unsubscribe always returns `{ ok: true }` after a valid email, whether or not the address was on the list.
Invalid emails still 400. That keeps the public form from being an email-oracle.