You'd want to be very careful trying to solder anything to the battery. You would probably blow it up if it got too hot. Better to try to solder the prong back in place after the battery is in. You could use a small bit of electrical or duct tape to hold it onto the battery. However, you might as well "bite the bullet" and start saving for a new board. Any fix will most likely be a temporary solution at best.
BTW, you are not alone. I did this once myself (long ago). Really embarrassing.