Of course don't tell them. It is always stupid to leave the money on the table. You win nothing by telling in any case. They will fire you, they will own your app because after all you are a programmer and you coded it while on the job so it belongs to them anyway, and others in that company will hate you because you hacked through shit they had to do manually for years. And they will even think that you have scammed them after all that, anyway.