A restored node whose source run is gone says its curve was lost

The run detail works out curve_lost per node when read, from whether its
cached_from run still exists. The run page names the deleted run in place
of the generic NO_CURVE note.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

backend/tests/api/routes/test_runs.py

@@ -1108,6 +1108,11 @@ def test_a_curve_whose_run_is_gone_is_empty_rather_than_an_error(
     )
     assert answer.status_code == 200
     assert answer.json() == []
+    # And the run says why, rather than leaving an empty chart to explain itself.
+    (node,) = client.get(
+        f"{settings.API_V1_STR}/runs/reuse-1", headers=superuser_token_headers
+    ).json()["nodes"]
+    assert node["curve_lost"] is True
 
 
 def _metric(run_id: str, name: str, step: int, value: float, ts: float) -> RunMetric: