Writer-director John Carney has become something of a cottage industry, making charming Dublin-set films about everyday people who love to make music whether it brings them fame or not. Movies like Once and Sing Street are designed to make you believe in the power of DIY music magic. They may be gentle pictures, but they remind you that just strumming a guitar can have transformative powers. They’re feel-good movies brushed with just the right amount of wistfulness. On the surface, Power Ballad