List active and scheduled temporary screen takeovers

Lists the workspace's temporary screen takeovers, latest start first, each with id, deviceId and deviceName, slideshowId and slideshowTitle, startsAt, endsAt (null when open-ended), cancelledAt and isActive (running right now). ?deviceId= narrows it to one screen; ?activeOnly=true returns only those running right now (not ended, cancelled or still to start). Use it to answer "is anything overriding the lobby screen?" or to find the id to end one early.