feat(music): add /music toggle and gate playback behind it

Music could be started and controlled with no way to turn it off for
a server. Adds a music_settings table (mirrors the same one
velox-dashboard-api's dashboard toggle reads/writes), a /music toggle
command gated to Manage Server, and checks in /play, /queue, and
/volume so they refuse to run when a server has music turned off.

internal/commands/commands.go

@@ -33,6 +33,7 @@ var AllCommands = []*discordgo.ApplicationCommand{
 	cmdmusic.Play,
 	cmdmusic.Queue,
 	cmdmusic.Volume,
+	cmdmusic.Music,
 	meeting.Meeting,
 	moderation.Moderation,
 	timezone.Timezone,
@@ -62,6 +63,7 @@ var handlers = map[string]func(s *discordgo.Session, i *discordgo.InteractionCre
 	"play":           cmdmusic.PlayHandler,
 	"queue":          cmdmusic.QueueHandler,
 	"volume":         cmdmusic.VolumeHandler,
+	"music":          cmdmusic.MusicHandler,
 	"config":         config.ConfigHandler,
 }