Join a member to a row’s thread

Put the member this credential acts for on the row’s thread, or change how they watch it (all, mentions, none). A participant’s role comes from how they joined, and this endpoint never changes it: send the role they already hold (as the participant list shows it) or, to join, the thread’s default role (member on a free-form table); any other role is a 403 FORBIDDEN. Another member’s place is never a credential’s to set (403 FORBIDDEN), whatever role the key carries. person must be an active member of this workspace — anyone else is a 422 MENTION_NOT_A_MEMBER.