B ella Kay is heading into a white-hot year. We’re seated in the back of Silence Please, a niche listening room-turned-café in lower Manhattan, just a few days after her 20th birthday. “I’ll never be on fire like I was when I was 19,” she jokes, quoting Lorde’s “Perfect Places.” Nineteen is a sacred age in pop. There’s something about it that rages and burns, especially among young women who process their emotions through melody and melodrama. When Olivia Rodrigo sang about 19 on “Teenage Dream,