Exactly 15 days ago, Lauren Betts knelt at center court of Mortgage Matchup Center in Phoenix, her face buried in her hands as confetti tumbled from above. The 6-foot-7 senior, a star clad in white, blue and gold, had just captured a national championship. Exactly 15 days later, as the Washington Mystics gathered for media day at CareFirst Arena on Monday, Betts donned a new red and white uniform, starring in an entirely different role: WNBA rookie.Subscribe to The Post Most newsletter for the m