No invented Resend key

Unsubscribe still deletes the row when RESEND_API_KEY is unset. Kumir never fakes a provider.

Kumir does not invent a Resend API key. If outbound email is unconfigured, unsubscribe still deletes NewsletterSignup locally.

When Resend is configured, marketing campaigns should include the /unsubscribe?email= link. That is not a fake success path around the list.