POST /team makes another team with the caller as owner. The website now
serves a team at nestri.io/<slug>, so Team.Slug refuses its top-level
pages (home, new, pricing, ...) and a personal team whose generated slug
would be one gets a suffix instead.
GET /team, PATCH /team/:id (owner or admin; a taken slug is a 409 on
slug), GET /team/:id/members, GET /box. A team you are not on is a 404
everywhere, so ids cannot be probed. Team.Name and Team.Slug are exported
so a client checks the same rule the API does.