CVImprover/cvimprover-api

AI Summary: This issue aims to enhance the user-friendliness of Django admin and debugging by adding or improving `__str__` methods for all models. The primary focus is on models within `core/models.py` and `cv/models.py`, including `User` and `Plan`, to ensure they have meaningful string representations. The task involves reviewing models, implementing the methods, and testing them in the Django admin.