`_db_compat.setup_db_compat()` should probably be called in `iommi.apps.AppConfig.ready`

[berycz]

I ran to it when adding SortOrderField into models.py, where I also had an abstract model
so when I did from iommi.models import SortOrderField in setup_db_compat, I got

  File "iommi/iommi/__init__.py", line 58, in <module>
    setup_db_compat()
  File "iommi/iommi/_db_compat.py", line 7, in setup_db_compat
    setup_db_compat_iommi()
  File "iommi/iommi/_db_compat.py", line 173, in setup_db_compat_iommi
    from iommi.models import SortOrderField
  File "iommi/iommi/models.py", line 17, in <module>
    class Orderable(models.Model):
  File "iommi/venv/lib/python3.12/site-packages/django/db/models/base.py", line 129, in __new__
    app_config = apps.get_containing_app_config(module)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "iommi/venv/lib/python3.12/site-packages/django/apps/registry.py", line 260, in get_containing_app_config
    self.check_apps_ready()
  File "iommi/venv/lib/python3.12/site-packages/django/apps/registry.py", line 138, in check_apps_ready
    raise AppRegistryNotReady("Apps aren't loaded yet.")

that's why I put the field in model_fields.py, but if it was called in app ready, we wouldn't have that problem