By Tom Balmforth and Jonathan Landay KYIV/KHARKIV, Ukraine (Reuters) – Even as the Kremlin prepares to take full control of the ruins of Mariupol city, it faces the growing prospect of defeat in its bid to conquer all of Ukraine’s eastern Donbas because its badly mauled forces lack the manpower for significant advances. Russian President Vladimir Putin may have to decide whether to send in more troops and hardware to replenish his dramatically weakened invasion force as an influx of modern Western weaponry bolsters Ukraine’s combat power, analysts say. Russia’s forces are unlikely to be vanqui…