(ui) Show retention apply details on run detail

Record planned delete counts, max-delete settings, base protection, and
ignored incomplete snapshots in retention apply results.

Surface those details on run detail pages so scheduled and manual prune
outcomes are understandable without reading the raw JSON payload.

src/pobsync_backend/tests/test_sql_retention.py

@@ -88,6 +88,9 @@ class SqlRetentionTests(TestCase):
             self.assertTrue(SnapshotRecord.objects.filter(pk=new.pk).exists())
             self.assertFalse(SnapshotRecord.objects.filter(pk=old.pk).exists())
             self.assertEqual(result["deleted"], [{"dirname": old.dirname, "kind": "scheduled", "path": str(old_dir)}])
+            self.assertEqual(result["planned_delete_count"], 1)
+            self.assertEqual(result["max_delete"], 1)
+            self.assertEqual(result["incomplete_ignored_count"], 0)
 
     def test_apply_deletes_snapshot_with_readonly_data_directory(self) -> None:
         with TemporaryDirectory() as tmp: