The fate of Sunday’s Super Bowl might not be determined by Brady’s arm or Gronk’s ankle. In fact, one California outplacement firm has calculated that the Patriots are the odds-on favorites against the Giants, based solely on the Boston area’s unemployment rate. RiseSmart Inc. ran the numbers for the past 20 Super Bowls. Of those 20 games, 17 have been won by the team whose metro area had a lower unemployment rate. RiseSmart helpfully notes that through November, the Boston metro area’s unemployment rate was 6.8 percent, compared with 8.5 percent in the New York metro area.