Invite an affiliate by email.

Invite one affiliate by email. Creates an affiliate login with an active membership for this brand and emails the affiliate their sign-in details; an affiliate this brand removed earlier is re-attached and re-invited. Refused when the brand is at its affiliate cap (400), for a disposable address (400), and for an address that already belongs to a user (409). Requires a signed-in brand owner or admin behind the key (the FUSE AI connector); an ordinary API key gets 403. Returns the membership id, its status, whether the email was sent and whether it was a re-invite.