By Guy Faulconbridge and Mark Trevelyan LONDON (Reuters) -President Vladimir Putin turned 70 on Friday amid fawning congratulations from subordinates and a plea from Orthodox Patriarch Kirill for all to pray for the health of the longest-serving paramount leader of Russia since Josef Stalin. Putin, who rose to the Kremlin’s top job on the last day of 1999, is facing the biggest challenge of his rule after the invasion of Ukraine triggered the gravest confrontation with the West since the 1962 Cuban Missile Crisis. His army there is reeling from a series of defeats in the past month. On his bir…