Fixes #19702: add NotificationGroup.event_rules GenericRelation

The collector we use to notify users about dependent object that will be
deleted does handle GFKs. However, a GenericRelation must be set up on
the other end.

netbox/extras/models/notifications.py

@@ -1,7 +1,7 @@
 from functools import cached_property
 
 from django.conf import settings
-from django.contrib.contenttypes.fields import GenericForeignKey
+from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation
 from django.core.exceptions import ValidationError
 from django.db import models
 from django.urls import reverse
@@ -144,6 +144,12 @@ class NotificationGroup(ChangeLoggedModel):
         blank=True,
         related_name='notification_groups'
     )
+    event_rules = GenericRelation(
+        to='extras.EventRule',
+        content_type_field='action_object_type',
+        object_id_field='action_object_id',
+        related_query_name='+'
+    )
 
     objects = RestrictedQuerySet.as_manager()