Ride it and be happy. There's a good chance the crack will not go anywhere. Sometimes cracks like that propagate until they get into less-stressed metal and then propagation stops and it can stay like that essentially forever. The way to find out is to ride it and see.

Repairing it is difficult and would ruin the paint, and this frame might not be worth such a Big Hairy Deal repair. Probably would mean replacing the seat tube. Should it eventually need a repair, and should you decide to go for it, riding it now will not make the eventual repair harder, so you have nothing to lose.

Just be glad you don't have a bulgie crack, those are nasty!

The artifact I'm seeing on the inside looks like maybe a seatpost was frozen and they sawed through it with a hacksaw and went a little too far? (aka a "hack job") Just a guess.

Mark B