RetrieveWithMediaFiles drops queue entries whose tracks are no longer in
the library (purged, or in a library the user lost access to), but it kept
the stored current index. That index then pointed one track further for
each dropped track before it, so getPlayQueue, getPlayQueueByIndex and
GET /api/queue reported the wrong song with the old position.
The current index is now remapped to the same track in the filtered list.
If the current track itself is gone, the queue moves to the next remaining
track (or the last one) and restarts it. The cleaned queue is then saved
back in the background, so partial PUT /api/queue updates use the same
list the client sees. The save is a conditional UPDATE that only applies
if the row is unchanged since it was read, so it never undoes a newer
client save, and it is skipped if loading the tracks failed.
Fixes#6297
Deluan
4fd394bead