(bugfix) Preserve existing schedule values in the edit form

Only apply default schedule initial values when creating a new schedule.

Avoid passing default initial data while editing an existing ScheduleConfig,
so the form renders the active cron-style expression, user, and retention
settings from the database.

Add a regression test that reopens an existing schedule and verifies the
stored values are shown instead of defaults.

src/pobsync_backend/views.py

@@ -535,7 +535,11 @@ def edit_host_schedule(request, host: str):
             messages.success(request, f"Schedule saved for {host_config.host}.")
             return redirect("host_detail", host=host_config.host)
     else:
-        form = ScheduleConfigForm(instance=schedule, initial=_default_schedule_initial())
+        form = (
+            ScheduleConfigForm(instance=schedule)
+            if schedule
+            else ScheduleConfigForm(initial=_default_schedule_initial())
+        )
 
     return render(
         request,