Who receives a newsletter. Needs audience, and never a public
surface: a newsletter's list belongs to its creator.
GET /subscriptions is that edge read from the other end, the lists one
account is on rather than the people on one list, and it needs
account: read instead. It carries none of what a newsletter's own record
of a subscriber carries: no address, no lifecycle status, none of the tags
the newsletter applied and nothing about how they were acquired.