Remove IDLE_CHECKER_ASSIGNMENT RBAC type and restrict assignment deletion to superadmin

Remove IDLE_CHECKER_ASSIGNMENT from EntityType/ScopeType/RBACElementType, the RBACElementTypeDTO mirror, the PermissionGQL.scope() match, and related tests. Change REST DELETE /idle-checker-assignments/{id} from auth_required to superadmin_required and add check_admin_only() to the GQL purge_idle_checker_assignment resolver. Keep the purge action scope-shaped but swap the RBAC validator for a superadmin validator, so audit keeps both the entity ref and the scope ref (a global-shape action would drop them).

## Success Criteria
- [ ] no IDLE_CHECKER_ASSIGNMENT references remain outside CHANGELOG and migrations
- [ ] non-superadmin with scope UPDATE: delete assignment returns 403; superadmin succeeds
- [ ] audit row for delete: operation DELETE, entity idle_checker:C1, scope ref intact
- [ ] component permission tests (test_scoped_search_permissions.py and friends) updated and passing
- [ ] pants test passes for affected packages

JIRA Issue: BA-7241

Locate the EntityType, ScopeType, RBACElementType, RBACElementTypeDTO, PermissionGQL.scope(), REST delete endpoint, and purge_idle_checker_assignment resolver named in the issue. Review the related component permission tests, including test_scoped_search_permissions.py, before changing authorization behavior. Done means the references and tests are updated, deletion is superadmin-only with an intact scope audit reference, and pants test passes for affected packages.

Written by the indexing model from the issue text.