Use the Teddy Roosevelt tactic of speak softly but carry a big stick (you already have one, since you're the engineer and the product is on your shoulders). As in, hint to them that you have sought legal counsel and that, if worse comes to worse, you wouldn't mind sudo rm -rf the entire code base. Be sure to be in the passive tone and say things like, "I had no choice but to be forced to seek advice from counsel", "this project means more to me than my life, and it saddens me greatly to see you guys do this to me", "please don't force me to take all time and effort I've put into this and go away forever", "I want to be working on fixing this and building a future, than crying at home and writing bitter memoirs", etc.
In general when speaking to them, highlight the fact you're the victim and (in the words of 4chan) the betafag. This is because you people are already in a highly emotional confrontation, and if everyone gets their backs up, chances are everyone except the lawyers will lose money (also, lawyers don't do anything except charge and tell you to settle, so you can't depend on them). The point is to act nice, but hint between the lines the lifeline of this business as well as the reputations of everyone involved is in your hands.
If the founders give in or compromise to your demands (which they should if you act like a reasonable human being who they have hurt and not an aggressive asshole out for revenge), let it all be water under the bridge - the founders have, no doubt, learned their lesson from this also and will improve in the future. But if they don't, don't sudo rm -rf the entire repo, instead, just write up your story, publish on HN, and go find work elsewhere (there is no sense in throwing good money after bad).