Is it obligatory to repeat the Fajr prayer after performing ghusl from janabah for someone who woke up after sunrise and found traces of semen on his underwear, knowing that he had prayed Fajr in the mosque before discovering the state of janabah?

You are not required to repeat the Fajr prayer. An impurity is attributed to the end of its time, not to what precedes it. The original presumption is that you were in a state of purity until you prayed Fajr, and the presence of a wet dream before that is doubtful. Certainty is not removed by doubt.