Unethical advice: move anything of value into your room, document her not locking the unit and request a bedroom lock for your room with your own key from your leasing office. Then without saying anything, take something valuable of hers and hide it in your room one day so she thinks she got robbed. Put it back overnight. She will be conditioned into locking the door.
If you had the energy and patience you could also fuck up the living room area a bunch to make it seem like someone came in and rifled through the stuff (if you don’t have cameras that is) I don’t want to assume anything about her but people who have never had to worry about money/belongings typically also don’t worry about stuff like locking the door. I get how frustrating it is, I’m sorry :(
thank god she at least brings her keys with her places now. she used to not to. one time she got mad at me bc i locked her out bc i didn't realize she wasn't home. i got back at like 10pm door was unlocked. so naturally i locked it behind me thinking she was home bc she hadn't told me she wasn't. then like 1 am rolls around shes literally complaining to our other roommate that i locked her out (caught it on the doorbell camera) literally said she'd been sitting out on the porch for over an hour