An estimated 321,000 Americans now work in the legal cannabis industry, outnumbering the country’s dentists, paramedics and electrical engineers. After a year on the front lines, Jason Zvokel traded in his 15-year career as a Walgreens pharmacist for a different kind of drugstore: a marijuana dispensary. Now instead of administering vaccines and filling prescriptions, he’s helping customers make sense of concentrates, tablets and lozenges. His pay is 5 percent lower, he said, but the hours…