Fix Check.lock_and_delete() to gracefully handle already deleted check

hc/api/models.py

@@ -346,7 +346,7 @@ def lock_and_delete(self) -> None:
         in the process of deletion.
         """
         with transaction.atomic():
-            Check.objects.select_for_update().get(id=self.id).delete()
+            Check.objects.select_for_update().filter(id=self.id).delete()
 
     def assign_all_channels(self) -> None:
         channels = Channel.objects.filter(project=self.project)

hc/api/tests/test_check_model.py

@@ -473,3 +473,11 @@ def test_get_status_handles_autumn_dst_transition(self) -> None:
             # The next expected run time is at 2023-10-29 01:15 UTC, so the check
             # should still be up for 10 minutes:
             self.assertEqual(check.get_status(), "up")
+
+    def test_lock_and_delete_handles_already_deleted_checks(self) -> None:
+        check = Check.objects.create(project=self.project)
+        same_check = Check.objects.get(id=check.id)
+        check.delete()
+
+        # lock_and_delete should handle an already deleted check gracefully:
+        same_check.lock_and_delete()