California's proposed billionaire tax appears headed for the November ballot after backers said they gathered more than 1.5 million signatures, well above the threshold needed to qualify. SF Standard reports: Backers of the initiative announced this weekend that more than 1.5 million people signed a petition to bring the one-time, 5% wealth tax to a statewide vote come November. That's well beyond the 875,000 names needed to qualify the measure, and likely sufficient to account for illegible or invalid signatures. The Service Employees International Union United Healthcare Workers West, a union representing more than 120,000 healthcare workers, pitched the tax to make up for federal spending cuts that threaten to shutter hospitals(opens in new tab) and kick millions of people off medical insurance. Proponents of California's wealth tax estimate it would raise $100 billion in one-time revenue, even if some billionaires leave because of the measure.…