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/music/registry.go

@@ -7,9 +7,10 @@ import (
 )
 
 var (
-	Play  *discordgo.ApplicationCommand = public.Play
-	Queue *discordgo.ApplicationCommand = public.Queue
+	Play   *discordgo.ApplicationCommand = public.Play
+	Queue  *discordgo.ApplicationCommand = public.Queue
 	Volume *discordgo.ApplicationCommand = public.Volume
+	Music  *discordgo.ApplicationCommand = public.Music
 )
 
 func PlayHandler(s *discordgo.Session, i *discordgo.InteractionCreate) { public.PlayHandler(s, i) }
@@ -17,4 +18,4 @@ func QueueHandler(s *discordgo.Session, i *discordgo.InteractionCreate) {
 	public.QueueHandler(s, i)
 }
 func VolumeHandler(s *discordgo.Session, i *discordgo.InteractionCreate) { public.VolumeHandler(s, i) }
-
+func MusicHandler(s *discordgo.Session, i *discordgo.InteractionCreate) { public.MusicHandler(s, i) }