Fix issue #190: [Claude] User model custom manager

[cld-vasconcelos]

This pull request fixes #190.

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

1. A custom NamedUserManager was correctly implemented that filters users based on non-empty name_first and name_last fields using proper Django query filters (excluding null and empty strings).

2. The manager was properly added to the User model as named_users while preserving the default objects manager, allowing both filtering methods to coexist without interference.

3. Comprehensive tests were added that:

   • Verify the named_users manager correctly returns only users with both names populated (test_named_users_manager)
   • Confirm the default manager still returns all users (test_default_manager_unchanged)
   • Include test cases for all relevant scenarios (null names, empty strings, populated names)

4. The implementation follows Django best practices by:

   • Properly extending models.Manager
   • Using appropriate query filtering (models.Q objects)
   • Maintaining existing functionality while adding new features

The changes directly address all requirements from the issue description and the tests demonstrate that both the new functionality works as intended and existing functionality remains unchanged. The code is properly structured and follows Django conventions for custom managers.

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