Fix issue #197: Add timestamps to models demo

[cld-vasconcelos]

This pull request fixes #197.

The issue has been successfully resolved based on the following implemented changes:

1. The timestamp fields were added correctly to the base SoftDeleteModel class, which means all models inheriting from it automatically receive these fields:

   • created_at (DateTimeField, null=True, auto_now_add=True)
   • updated_at (DateTimeField, null=True, auto_now=True)

2. A proper migration file (0005_collection_created_at...) was created that adds these fields to all existing models in the system, including Collection, Contest, ContestSubmission, Picture, PictureComment, and User.

3. The implementation matches the requirements exactly:

   • Both fields are nullable (null=True specified)
   • The fields are automatically populated (auto_now_add and auto_now)
   • All models receive these fields through inheritance

4. Comprehensive tests were added that verify:

   • Fields are properly set on creation
   • updated_at changes when models are modified
   • Fields contain the expected datetime values

The changes are complete and functional, providing the requested timestamp tracking functionality across all models while maintaining the requirement for nullable fields.

Automatic fix generated by OpenHands 🙌

[cld-vasconcelos]

@openhands-agent in types.py file add the created_at and updated_at fields to all classes that do not have them.

[cld-vasconcelos]

OpenHands started fixing the pr! You can monitor the progress here.

[cld-vasconcelos]

Summary of Changes:

• Added created_at and updated_at fields to all model types in types.py
• Made timestamp fields nullable using strawberry.auto | None
• Implemented consistently across all models

Status: ✅ All issues successfully resolved

• All models now have proper timestamp tracking
• Nullable requirements met
• Implementation is consistent across the codebase

No remaining issues identified - the changes fully address the original requirements.