Question: During wintertime it is often cold enough in my apartment that proofing takes much longer than in summer. I'd like to use my oven to create a warmer environment, in which the dough would rise faster. I can go 30-50 degrees celsius, the question is what temperature is safe and at what temperature will I actually start baking the dough. Is 50 degrees celsius still ok or not?

Answer: 
50C (122F) would be a very high proofing temperature. The thermal death point of yeast is 55C, and you'll definitely hit a point of diminishing returns if you get too hot (most likely, you will have really rapid proofing on the outside of the loaf and an underproofed "core"). 

I would recommend setting your oven to the lowest temperature, and then once it feels perceptibly warm, turn it off and use it to proof. If you're doing a long proof, you can turn it back on for a minute or two every so often, but I wouldn't ever let it actually get up to temperature.
Rise from chilled @ 32C and keep rising until around 80-85% of desired height has been achieved. 
Return back to the chiller, it will continue to rise for a short time, keep in the chiller until dough is firm again and stable enough to take the weight of toppings.