(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)
 

src/pobsync_backend/templates/pobsync_backend/host_detail.html

@@ -49,7 +49,8 @@
       <h2>Schedule</h2>
       {% if schedule %}
         <div class="stack">
-          <div><strong>Cron:</strong> {{ schedule.cron_expr }}</div>
+          <div><strong>Schedule expression:</strong> {{ schedule.cron_expr }}</div>
+          <div class="muted">Evaluated by the pobsync scheduler service.</div>
           <div><strong>Enabled:</strong> {{ schedule.enabled|yesno:"yes,no" }}</div>
           <div><strong>Prune:</strong> {{ schedule.prune|yesno:"yes,no" }}</div>
           <div><strong>Last status:</strong> {{ schedule.last_status|default:"" }}</div>

src/pobsync_backend/templates/pobsync_backend/schedule_form.html

@@ -11,6 +11,7 @@
 
   <section class="panel">
     <h2>{% if schedule %}Edit Schedule{% else %}Create Schedule{% endif %}</h2>
+    <p class="muted">Schedules use cron-style timing syntax, but they are evaluated by the pobsync scheduler service.</p>
     <form method="post" class="form-grid">
       {% csrf_token %}
       {{ form.non_field_errors }}

src/pobsync_backend/tests/test_views.py

@@ -546,6 +546,8 @@ class ViewTests(TestCase):
         self.assertContains(response, "web-01")
         self.assertContains(response, "web-01.example.test")
         self.assertContains(response, "15 2 * * *")
+        self.assertContains(response, "Schedule expression")
+        self.assertContains(response, "Evaluated by the pobsync scheduler service.")
         self.assertContains(response, "20260519-021500Z__ABCDEFGH")
         self.assertContains(response, "Discover snapshots")
         self.assertContains(response, "Edit schedule")
@@ -1135,6 +1137,8 @@ class ViewTests(TestCase):
 
         self.assertEqual(response.status_code, 200)
         self.assertContains(response, "Create Schedule")
+        self.assertContains(response, "Schedule expression")
+        self.assertContains(response, "evaluated by the pobsync scheduler service")
         self.assertContains(response, "15 2 * * *")
         self.assertContains(response, "Save schedule")