(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/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")