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/repos/settingsrepo/repo.go

@@ -340,3 +340,22 @@ func (r *Repo) GetLevelUpMessage(ctx context.Context, guildID int64) (string, er
 		return "", err
 	}
 }
+
+func (r *Repo) IsMusicEnabled(ctx context.Context, guildID int64) (bool, error) {
+	const q = `SELECT is_enabled FROM music_settings WHERE guild_id = $1`
+	var enabled bool
+	err := r.db.QueryRowContext(ctx, q, guildID).Scan(&enabled)
+	if err == sql.ErrNoRows {
+		return false, nil
+	}
+	if err != nil {
+		return false, err
+	}
+	return enabled, nil
+}
+
+func (r *Repo) SetMusicEnabled(ctx context.Context, guildID int64, enabled bool) error {
+	const q = `INSERT INTO music_settings (guild_id, is_enabled) VALUES ($1, $2) ON CONFLICT (guild_id) DO UPDATE SET is_enabled = $2`
+	_, err := r.db.ExecContext(ctx, q, guildID, enabled)
+	return err
+}