DETROIT — The case of an overnight attack on a couple in a small Michigan town could rewrite the rules for life sentences for teens convicted of murder.The state appeals court said juries — not judges — must decide whether someone under 18 gets life in prison without parole — an extraordinary shift in Michigan’s justice system.Juries determine guilt at trials, of course, but punishment has been reserved for judges. An appeal to the state Supreme Court is certain.“This is certainly a …