Stop sending emails (sets active=0). Does NOT touch billing.

Unsubscribing and cancelling are different things and used to be the same button: clicking 'unsubscribe' in an email footer silently cancelled a $9/mo subscription the person had not asked to end. Now this stops mail only, and a paying subscriber is TOLD their billing is still running with a link to cancel it separately.

token is the subscriber's api_key. Idempotent; unknown tokens get a generic friendly page.