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/play.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"net/url"
 	"strings"
+	"velox-bot/internal/commands/music/shared"
 	"velox-bot/internal/music"
 
 	"github.com/bwmarrin/discordgo"
@@ -23,6 +24,10 @@ var Play = &discordgo.ApplicationCommand{
 }
 
 func PlayHandler(s *discordgo.Session, i *discordgo.InteractionCreate) {
+	if !shared.RequireMusicEnabled(s, i) {
+		return
+	}
+
 	data := i.ApplicationCommandData()
 	if len(data.Options) == 0 {
 		return