There is a lengthy discussion in the comments already, and I hope people more familiar with the German side of things have sufficiently elaborated on that.
I would instead try to offer you an analysis from the perspective of someone still planning the move.
You have a pool of resources available to you and a bunch of liabilities. Your biggest liabilities are:
- Money. You need money to survive, at the end of the day, and will need to sort it out somehow. From your post, you do not have much left and spending an year or two on tropical island vacation arranging your next position is understandably not on the list.
- Legal status/background. Goes without saying.
- Poor social capital. You do not seem to have a strong network, especially outside of Russia. Your research profile is okay, but not stellar.
- Doing anything outside of continuing research is clearly not optimal in the long run, but you might need to make concessions as you go.
Your main recourse lies in whatever reputable contacts you do have and in what skills you demonstrably have. In this case, it seems like the professor has jumped the gun and decided to avoid any potential complications - after all, they have not that much to gain and a lot to lose. That you have not managed to make a convincing case in over 4 months since the date your contract was supposed to start is hugely problematic. I am afraid convincing the university to reverse the decision within a week is, essentially, an impossible task.
Given the visa expiration date, your immediate next step would be either moving somewhere else for the time being (this may prove impossible due to money issues), applying for the asylum or getting stuck in the legal limbo. Like @Dr.Snoopy points out, you could, in theory, live in Germany without having the paperwork and try to find employment, but this is also problematic for obvious reasons. I would advice at least consider applying for the asylum: that way your stay will be legal while your application is being considered (of course, it is not of much help if you get a desk reject the very next day).
Another immediate action is contacting whoever you believe could support you one way or another - your advisor and colleagues are probably your first option here when it comes to job seeking, but pursue as many leads as you possibly can at once.
Finally, have a plan of action for when things go wrong and do not put all eggs in one basket (I guess you have learned that lesson the hard way). Your current situation is fairly bad, so adjust your expectations accordingly - it will give you a different perspective on your options.
It is soul crushing to have a bright future taken away from you by something you did not really have influence over, but, IMHO, it is better to view your circumstances pessimistically and be ready to work hard and build your career from the ground up. On the plus side, you do not have to leave behind some 20+ years of your work.
I sincerely hope you will be able to turn it around.
UPD: Things seem to get increasingly complicated with respect to being able to move outside of Russia once you are here. Treat the returning option as your very last resort, and maybe not even that.