bugfix blocking on repeated song request enqueue during shutdown

[nickwitten]

Song request enqueue was blocking main loop for power board as the shutdown song was repeatedly enqueued and only dequeued after each song was completed.

Removed redundant request in the coms task.

Changed spam song request enqueue to periodic enqueue.

[guyfleeman]

could just remove _audio_publisher unless there's a strong future need for it. Fix seems reasonable tho. I think in general a publish().await is almost always wrong unless its related to a param update maybe.