By day, Shenzhen flaunts its role as the high-tech factory for the world.Self-driving taxis collect passengers from the roadside, drones carry parcels, and delivery robots jostle with passengers in hotel elevators. By night, the city’s factories keep running. The high-rise offices stay lit, the traffic thins out but still flows, and crowds are drawn to streetside food stalls.Just after 10 pm at a farmers’ market in Longgang district, an hour’s drive from downtown, grocery store manager Zhang Chao stands amid the bustle of produce stalls, pulls out his phone and summons a robot. What arrives a few moments later is…