SAO PAULO, Brazil (Reuters) - Cuban authorities have told the Brazilian government that Cuban leader Fidel Castro has a malignant stomach tumor and may be unable to return to full power, a Brazilian newspaper reported on Saturday. Folha de S.Paulo said Brazilian officials had been told that Castro's illness marked the start of an irreversible transfer of power on the Communist-ruled island. Folha said Cuban authorities informed Brazilian President Luiz Inacio Lula da Silva and members of the ruling Workers' Party about the malignant stomach tumor this week. His condition was worse than has been publicly admitted, according to Folha....