Lucas Root says he doesn’t remember seeing the door that opened up in front of his bike, knocking him to the pavement and breaking out three teeth. But when he learned the driver was working for Uber, the app-based ride-hailing service, he saw an insurance nightmare ahead. Root had worked briefly for an on-demand bicycle delivery service whose couriers use their own bicycles, so he knew that the so-called “gig economy” tends to muddy issues of labor rules and who’s responsible when things go wrong.