Sarah lives in a walk-up apartment in Brooklyn. Third floor. Shared hallway. No doorman. Like most renters in New York City, she has a constant low-grade anxiety about package theft, unexpected visitors, and the fact that her landlord has a master key to her front door. "I love my apartment," she told me. "But I never felt completely safe. Anyone could buzz any unit and get into the building. And once they're in the hallway, my door is right there." Last winter, someone tried her door handle at 3 AM. The deadbolt held. But the next...