When Elizabeth Matthews thinks about the most unforgettable concerts of her life, her mind goes back to Ann Arbor, Mich., in February 1983, when her high school basketball coach scored free tickets for the whole team to see Prince on his 1999 tour. But there was a catch. “She agreed that we would all be the janitors after the show,” Matthews recalls. “We got to see Prince for free, and then we had to clean up the Crisler [Center] with our brooms.” Matthews was happy to pitch in — and it proved to be an early sign of the work she’s willing to do behind the scenes for songwriters.…