Check whether a screen is online and what it shows

Returns one screen's online (true when it checked in within the last few minutes), lastSeenAt, and nowPlaying.slideshowId (what its schedule and takeovers resolve to now; nowPlaying is null when nothing is assigned). A quick check after pairing or a change. For all screens at once use GET /v1/workspace/health; for why a screen looks wrong, GET /v1/devices/{id}/diagnosis.