SAN DIEGO — In a typical last-minute scramble, Jannette Navarro, a 22-year-old Starbucks barista and single mother, scraped together a plan for surviving the month of July without setting off family or financial disaster.

In contrast to the joyless work she had done at a Dollar Tree store and a KFC franchise, the $9-an-hour Starbucks job gave Ms. Navarro, the daughter of a drug addict and an absentee father, the hope of forward motion. She had been hired because she showed up so many times, cheerful and persistent, asking for work, and she had a way of flicking away setbacks — such as a missed bus on her three-hour commute — with the phrase, “I’m over it.”