Fix manage.py delete-user failing on users with SeenMessage records

[heathdutton]

Fixes #6616

manage.py delete-user fails with an IntegrityError when deleting a user that has SeenMessage records. This happens because it calls db.session.delete() directly instead of using the Journalist.delete() method, which properly handles related records.

This change updates manage.py to use Journalist.delete(), matching the behavior of the Admin Interface.

Test plan

1. Create a user via manage.py add-journalist
2. Log in as that user via SecureDrop Client and read some messages (creating SeenMessage records)
3. Run manage.py delete-user and enter the username
4. Verify the user is deleted successfully without errors

Checklist

This change accounts for:

• any required additional documentation
• any necessary AppArmor changes (added or removed application files)
• any impact on new SecureDrop installs and upgrades
• our dependency update policy