GENEVA (Reuters) – The wife of a Ukrainian fighter imprisoned by Russia called for his release on Tuesday as a U.N. rights official said monitors had documented cases of Ukrainian prisoners being tortured, in two cases to death. Some Ukrainian soldiers who fought in a weeks-long siege in the battle for the city of Mariupol before eventually surrendering in May were released last month. However, others who were holed up in the besieged Azovstal steelworks in the port city remain captive, including the husband of Liliia Stupina, a 24-year-old former data research analyst who left her job to seek…