Fix IntegrityError when creating periodic tasks

Change PeriodicTask.objects.get_or_create() to PeriodicTask.objects.update_or_create().

This change prevents a potential IntegrityError when creating a periodic task due to duplicate primary key. By using update_or_create, if the record already exists, it will be updated instead of attempting to insert a new record with the same primary key, ensuring the process completes without error.

bookwyrm/views/admin/dashboard.py

@@ -83,7 +83,7 @@ def post(self, request):
             schedule, _ = IntervalSchedule.objects.get_or_create(
                 **schedule_form.cleaned_data
             )
-            PeriodicTask.objects.get_or_create(
+            PeriodicTask.objects.update_or_create(
                 interval=schedule,
                 name="check-for-updates",
                 task="bookwyrm.models.site.check_for_updates_task",