CachedScopeMixin._region and ._site_group may cache ancestors of a
Site or Location scope. Change these relationships to SET_NULL so
deleting a Region or SiteGroup clears the cached value instead of
deleting the scoped Prefix, Cluster, or WirelessLAN.
Add reverse GenericRelation fields for Cluster and WirelessLAN on
Region and SiteGroup. This preserves the expected cascade when a
Region or SiteGroup is itself the direct scope, matching the existing
Prefix behavior.
Add migrations recording the ORM-level on_delete changes and regression
coverage for Site, Location, and direct Region/SiteGroup scopes.
0eb1fcc09c