Fix issue #79: test created_at and updated_at

[cld-vasconcelos]

This pull request fixes #79.

The issue has been successfully resolved. The changes made include adding created_at and updated_at fields to each model in the application. These fields are defined as DateTimeField with auto_now_add=True and auto_now=True, respectively, and are nullable, which aligns with the requirements. Additionally, a new abstract model TimestampedModel was created, which includes these fields, and other models like SoftDeleteModel now inherit from it, ensuring consistency across models. Furthermore, tests were added to verify that each model has these timestamp fields and that they are not None upon object creation, confirming their presence and functionality. These changes collectively address the issue as described.

Automatic fix generated by OpenHands 🙌