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/db/services/musicsettings/service.go

@@ -0,0 +1,32 @@
+package musicsettings
+
+import (
+	"context"
+	"velox-bot/internal/db/repos/settingsrepo"
+)
+
+type Service struct {
+	settings *settingsrepo.Repo
+}
+
+func New(settings *settingsrepo.Repo) *Service {
+	return &Service{settings: settings}
+}
+
+func (s *Service) ToggleMusic(ctx context.Context, guildID int64) (bool, error) {
+	enabled, err := s.settings.IsMusicEnabled(ctx, guildID)
+	if err != nil {
+		return false, err
+	}
+
+	newEnabled := !enabled
+	if err := s.settings.SetMusicEnabled(ctx, guildID, newEnabled); err != nil {
+		return false, err
+	}
+
+	return newEnabled, nil
+}
+
+func (s *Service) IsMusicEnabled(ctx context.Context, guildID int64) (bool, error) {
+	return s.settings.IsMusicEnabled(ctx, guildID)
+}