(refactor) Clarify scheduler terminology in the Django UI

Rename the schedule form label from "Cron expression" to "Schedule
expression" and explain that cron-style timing is evaluated by the
pobsync scheduler service rather than host cron.

Update host detail and schedule form copy so operators can see that
schedules are SQL-backed and dispatched by pobsync itself.

src/pobsync_backend/forms.py

@@ -265,8 +265,11 @@ class RetentionApplyForm(forms.Form):
 
 class ScheduleConfigForm(forms.ModelForm):
     cron_expr = forms.CharField(
-        label="Cron expression",
-        help_text='Five-field cron expression, for example "15 2 * * *".',
+        label="Schedule expression",
+        help_text=(
+            'Five-field cron-style expression, for example "15 2 * * *". '
+            "This is evaluated by the pobsync scheduler service, not host cron."
+        ),
     )
     prune_max_delete = forms.IntegerField(min_value=0)