DETROIT — Fiat Chrysler must offer to buy back from customers more than 500,000 Ram pickup trucks and other vehicles in the biggest such action in U.S. history as part of a costly deal with safety regulators to settle legal problems in about two dozen recalls.The Italian-American automaker also faces a record civil fine of up to $105 million. In addition, owners of more than a million older Jeeps with vulnerable rear-mounted gas tanks will be able to trade them in or be paid by Chrysler to have the vehicles repaired.The …