Retrieve a chat room

Read one chat room by id, including whether its creator has hidden it (hidden). Needs audience: read, on a newsletter the credential reaches.

Live rooms only, which is where this differs from GET /tags/{tag}: that operation resolves a retired tag on purpose, because a client rendering the audience of an article sent last year holds the id and needs the name. A room that was removed, or whose tag was retired, is not a room, and answers 404 here. Read it as a tag instead.

A hidden room does resolve, carrying hidden: true.