The New York Jets are hoping that their most prominent addition this offseason — a former most valuable player of the Super Bowl — never takes a snap for them. Such is the plight of a backup quarterback, and that is the new role for veteran Joe Flacco, who agreed to a one-year deal Friday as he recovers from neck surgery. If having a reliable understudy at football’s most important job can preserve a season — just ask New Orleans, which won all five games last year behind Teddy Bridgewater after an injury to Drew Brees, or Philadelphia, which won a championship with Nick Foles — then undervaluing that position can derail it. The Jets endured this reality last season when they lost all three games while their starter, Sam Darnold, was out with mononucleosis and fell to 0-4.