What is the code to the safe?

The code is

4R3UHER3

If you ordered the special edition and got the physical items or go to this page here, you can get the code out of the file names.

If you bought the Steam version, the code (same as given as answer above) is given to you within the pdf-manual that comes along with the game. It's a bit hidden but I'm sure you will see it. Can't find the pdf? View the game in Steam store, scroll down, and in the right column you will se the link "View the manual".