Fixes: 1) Add missing import for `transaction` in `signals.py`; 2) Fix incorrect handling of `pre_save` skip logic for non-existent users; 3) Correct conditional logic for email change handling to avoid unnecessary activation resets.
Extra: 1) Simplify mutation attribute validation in `mutations.py` by excluding specific fields earlier in the loop; 2) Improve code readability with minor refactors in both `signals.py` and `mutations.py`.
