Create and update contacts in Resend (segments, unsubscribe handling)

Documented steps

1. POST https://api.resend.com/contacts with { email } (required); optional: first_name, last_name, unsubscribed (bool), properties (custom key/value object), segments ([{id}]), topics ([{id, subscription: 'opt_in'|'opt_out'}])
2. Response: { object: 'contact', id: '<uuid>' }
3. Update with PATCH https://api.resend.com/contacts/{id_or_email} — the path accepts either the contact UUID or the email address; partial updates supported
4. Set unsubscribed: true to suppress the contact from all future broadcast sends