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 concrete changes:

1. The required timestamp fields were added to the SoftDeleteModel class, which serves as the base class for all models:

   • created_at: DateTimeField with null=True and auto_now_add=True
   • updated_at: DateTimeField with null=True and auto_now=True

2. A proper migration file (0005_collection_created_at_collection_updated_at_and_more.py) was created that adds these fields to all relevant models in the system:

   • Collection
   • Contest
   • ContestSubmission
   • Picture
   • PictureComment
   • User

The changes directly address the requirements:

• Both fields are nullable (null=True specified)
• The fields will automatically track creation and update times (auto_now_add and auto_now)
• The fields have been added to all models through inheritance from SoftDeleteModel
• The migration file contains all necessary operations to add these fields to existing tables

The implementation is complete and correct - when applied, these changes will add the required timestamp tracking to all models in the system while maintaining data integrity by making the fields nullable as specified.

Automatic fix generated by OpenHands 🙌