Brynn Putnam has a lot to feel great about. A Harvard grad and former professional ballet dancer who opened the first of what have become three high-intensity fitness studios in New York, she then launched a second business in 2016 when — while pregnant with her son — she was exercising at home and couldn’t find a natural way watch a class on her laptop or phone. Her big idea: to install mirrored screens in users’ homes that are roughly eight square feet and through which they can exercise to all manner of streamed and on-demand exercise classes, paying a monthly subscription of just $39 per month.