Fix case sensitivity issue when retrieving user by username

The commit fixes a bug where the user object was not found due to case sensitivity in the username. Now, the username is converted to lowercase before querying the database.

backend/api/routes/mfa_api.py

@@ -451,7 +451,7 @@ def post(self):
                 return {"message": "No username found"}, 400
 
             # Get the user from the database
-            user = Accounts.get_or_none(Accounts.username == username)
+            user = Accounts.get_or_none(Accounts.username == username.lower())
 
             # Check if the user exists
             if not user: