fix(web): backup age in Templates card was frozen by hx-preserve

The backups <details> carried hx-preserve="true" to survive the card's
30s poll without collapsing if the operator had it open -- but hx-preserve
freezes the whole node, so "newest X ago" inside it never updated either,
same class of bug as the Started column before it got its own JS-side
toggle state. Drop hx-preserve; track open/closed the same way (stable id,
state re-applied after every swap) so the count/age text stays live.

tests/python/test_web_stage_b.py

@@ -180,6 +180,38 @@ def test_templates_route_degrades_on_probe_failure(
     assert "unavailable" in resp.text.lower()
 
 
+def test_templates_backups_details_is_not_frozen_by_hx_preserve(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """``hx-preserve`` freezes the *whole* node across the card's 30s poll --
+    including the "newest X ago" text inside it, which must keep updating.
+    Only the <details> open/closed state should persist (via app.js, the
+    same pattern already used for the Started column's click-to-toggle)."""
+    monkeypatch.setattr(app_module, "list_templates", lambda _config: [_TEMPLATE_ROW])
+    monkeypatch.setattr(
+        app_module,
+        "backups_by_template",
+        lambda _config: {
+            "LinuxBuild2404": [
+                FileEntry(
+                    name="Template_LinuxBuild2404_20260101_000000.7z",
+                    size_bytes=2048,
+                    mtime=1_700_000_000.0,
+                )
+            ]
+        },
+    )
+    app = create_app(_make_config(tmp_path))
+    client = TestClient(app)
+
+    resp = client.get("/templates")
+
+    assert resp.status_code == 200
+    assert 'id="backups-LinuxBuild2404"' in resp.text
+    assert "newest" in resp.text
+    assert "hx-preserve" not in resp.text
+
+
 def test_templates_route_shows_running_ram_snapshot_warning(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None: