recently rehabbed starlet Demi Lovato toasted her 19th birthday with yogurt and a “Sober Is Sexy”-themed bash. Sources said the star indulged only in yogurt at the Yogen Fruz bash at Beacher’s Madhouse in Los Angeles on Friday. But hours later, at 2 a.m., she was spotted at an LA convenience store with a mystery man who bought a 12-pack of Miller Lite. Lovato entered a facility for emotional and physical issues in November. Her rep didn’t get back to us.