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.
I've done this a few times, kinda - we have a baking stone, and I'll preheat that, make sure it's cool enough to touch, and set the dough on the warmed stone. 

It works pretty well to give a warm environment, the warmth lasts because the closed oven contains heat, and there's little to no risk of overheating the yeast. I have both gone with regular preheating for ~5min, and the lowest-temperature-preheating for a bit longer, the former is quicker to heat but more likely to overshoot (or needing time to even the heat out), the latter slower and more controlled.

Another *potential* alternative, depending on your oven, might be to turn the oven on (bake setting, usually) but keep the temperature set to zero. In our oven, that turns on the light, the machinery is humming, and the oven gets kinda warm - not hot, but just warm, preparing to heat but not doing so. 

I've used this to dehydrate things before, because I can leave it going for quite some time without needing to re-check or re-warm. It wouldn't work for me with bread dough, since with aforementioned baking stone it would take longer to warm up than it would for the dough to actually rise (since especially the bottom of the dough, against the stone, will rise very differently from the top) . But if you don't have a baking stone it may work very well for you.