UpNextQueue should not expose the Playable object

From the original issue report:

> Doing this makes it possible for somewhere else to change the episode's properties. Exposing a uuid would be fine or expose an automatically updating version.

Start by locating UpNextQueue and all call sites that access its Playable object. Review how callers use the episode properties and compare the issue's suggested UUID or automatically updating version. Done means the queue no longer exposes a mutable Playable object while callers retain the required episode identity or updates; the issue names no tests or files to run.

Written by the indexing model from the issue text.