The dashboard has been writing skip/pause/volume/etc. requests into
music_commands with nothing on this end reading them. Adds a
dedicated LISTEN connection (can't use the pooled *sql.DB for this,
LISTEN has to stay bound to one specific connection) that wakes on
NOTIFY, drains unprocessed rows, and dispatches each to the matching
Manager function. A command's error is logged, not fatal, and every
row gets marked processed regardless of outcome so a permanently
broken command doesn't retry forever.
FernandoJVideira
fcc7c9c66a