What is the ruling on the prayer of the Imam and the congregants when the Imam forgets a prostration in the first rak'ah, then completes the prayer and performs the prostration of forgetfulness after the final tashahhud?

The congregants were supposed to alert the Imam by saying "Subhan Allah" when he forgot the prostration.

If the Imam realized his mistake before he began reciting in the second rak'ah, he should return and perform the prostration, then complete his prayer and perform the prostration of forgetfulness (sujud al-sahw).

If he did not realize until after he began reciting in the second rak'ah, the rak'ah in which he forgot the prostration becomes void, and the second rak'ah becomes the first. He should then perform the second rak'ah and prostrate for forgetfulness.

Since he did not do any of that, despite remembering that he had omitted the prostration, his prayer is invalid, and he must repeat it. The same applies to those congregants who followed him, because prostration is a pillar of prayer that cannot be rectified by the prostration of forgetfulness (sujud al-sahw) except by performing it.