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/public/volume.go

@@ -2,6 +2,7 @@ package public
 
 import (
 	"fmt"
+	"velox-bot/internal/commands/music/shared"
 	"velox-bot/internal/music"
 
 	"github.com/bwmarrin/discordgo"
@@ -23,6 +24,9 @@ var Volume = &discordgo.ApplicationCommand{
 }
 
 func VolumeHandler(s *discordgo.Session, i *discordgo.InteractionCreate) {
+	if !shared.RequireMusicEnabled(s, i) {
+		return
+	}
 	_ = s.InteractionRespond(i.Interaction, &discordgo.InteractionResponse{
 		Type: discordgo.InteractionResponseDeferredChannelMessageWithSource,
 		Data: &discordgo.InteractionResponseData{
@@ -60,4 +64,3 @@ func VolumeHandler(s *discordgo.Session, i *discordgo.InteractionCreate) {
 }
 
 func ptrFloat(v float64) *float64 { return &v }
-