Fix issue #192: [Claude] User groups

[cld-vasconcelos]

This pull request fixes #192.

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

1. The many-to-many relationship requirement was properly implemented through:

• Creation of the Group model with appropriate fields
• Creation of the UserGroup intermediate model with required foreign keys
• Proper setup of the many-to-many relationship via the members field on Group model

2. The additional fields requirement was met by adding:

• role field in UserGroup model
• joined_at field with auto_now_add in UserGroup model

3. Data integrity is ensured through:

• Proper foreign key relationships
• A unique constraint preventing duplicate user-group combinations
• Inheritance from SoftDeleteModel maintaining consistent deletion behavior

4. The implementation is thoroughly tested with new tests covering:

• Basic CRUD operations
• Relationship constraints
• Multiple user/group scenarios
• Soft deletion functionality

5. The migration file properly sets up all necessary database changes without affecting existing models.

The changes directly address all requirements in the issue description while maintaining existing functionality through proper model inheritance and constraint management. The comprehensive test suite verifies the implementation works as intended.

Automatic fix generated by OpenHands 🙌