(ui) Surface retention warnings on run detail

Show host-level retention warnings on run detail pages so successful or
warning runs still expose scheduled prune limit issues and incomplete
snapshots that need operator attention.

src/pobsync_backend/tests/test_views.py

@@ -1213,6 +1213,37 @@ class ViewTests(TestCase):
         self.assertContains(response, "Retention")
         self.assertContains(response, "Deletion blocked by --max-delete=0")
 
+    def test_run_detail_surfaces_host_retention_warnings(self) -> None:
+        self.client.force_login(self.staff_user)
+        host = HostConfig.objects.create(
+            host="web-01",
+            address="web-01.example.test",
+            retention_daily=0,
+            retention_weekly=0,
+            retention_monthly=0,
+            retention_yearly=0,
+        )
+        ScheduleConfig.objects.create(host=host, cron_expr="15 2 * * *", enabled=True, prune=True, prune_max_delete=1)
+        self._snapshot(host, "20260517-021500Z__OLDSNP1")
+        self._snapshot(host, "20260518-021500Z__OLDSNP2")
+        self._snapshot(host, "20260519-021500Z__NEWSNAP")
+        SnapshotRecord.objects.create(
+            host=host,
+            kind=SnapshotRecord.Kind.INCOMPLETE,
+            dirname="20260519-031500Z__BROKEN01",
+            path=f"/backups/{host.host}/.incomplete/20260519-031500Z__BROKEN01",
+            status="failed",
+            started_at=datetime(2026, 5, 19, 3, 15, tzinfo=timezone.utc),
+        )
+        run = BackupRun.objects.create(host=host, status=BackupRun.Status.SUCCESS, result={"ok": True})
+
+        response = self.client.get(reverse("run_detail", args=[run.id]))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "Retention Warnings")
+        self.assertContains(response, "Scheduled pruning for this host would delete 2 snapshot(s)")
+        self.assertContains(response, "1 incomplete snapshot(s) exist for this host")
+
     def test_run_detail_infers_rsync_log_from_snapshot_path(self) -> None:
         self.client.force_login(self.staff_user)
         host = HostConfig.objects.create(host="web-01", address="web-01.example.test")