Remove resolved_by_alert field from state and remove constraint (#5156)

Removes resolved_by_alert field from Django state only. It will still be
in db, since it's to expensive to delete it in prod.
We remove this field to get rid of ON_DELETE=SET_NULL which causes
terrible load from SQL queries.
On top of that, we are removing FK to disable referral integrity checks,
so alert updates will work.

---------

Co-authored-by: Vadim Stepanov <[email protected]>

engine/apps/alerts/migrations/0061_alter_alertgroup_resolved_by_alert.py

@@ -0,0 +1,19 @@
+# Generated by Django 4.2.15 on 2024-10-10 11:02
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('alerts', '0060_relatedincident'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='alertgroup',
+            name='resolved_by_alert',
+            field=models.ForeignKey(db_constraint=False, default=None, null=True, on_delete=django.db.models.deletion.DO_NOTHING, related_name='resolved_alert_groups', to='alerts.alert'),
+        ),
+    ]

engine/apps/alerts/models/alert_group.py

@@ -289,7 +289,8 @@ class AlertGroup(AlertGroupSlackRenderingMixin, EscalationSnapshotMixin, models.
 
     resolved_by_alert = models.ForeignKey(
         "alerts.Alert",
-        on_delete=models.SET_NULL,
+        on_delete=models.DO_NOTHING,
+        db_constraint=False,
         null=True,
         default=None,
         related_name="resolved_alert_groups",