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/fluksio/api/routes/runs.py

@@ -146,6 +146,9 @@ class RunNodeRow(BaseModel):
     #: Which run it was restored from, when it was. That run is also where this
     #: node's series was recorded.
     cached_from: str = ""
+    #: That run has since been deleted, and this node's curve went with it. Its
+    #: outputs are still on this run's result.
+    curve_lost: bool = False
 
 
 class ArtifactRow(BaseModel):
@@ -781,6 +784,16 @@ def read_run(run_id: str, session: SessionDep) -> Any:
     ).all()
     detail = RunDetail.model_validate(run, from_attributes=True)
     detail.nodes = [RunNodeRow.model_validate(n, from_attributes=True) for n in nodes]
+    # Worked out here, not stored: a source run is deleted long after the
+    # nodes restored from it were written.
+    sources = {n.cached_from for n in nodes if n.cached_from}
+    kept = (
+        set(session.exec(select(col(Run.id)).where(col(Run.id).in_(sources))))
+        if sources
+        else set()
+    )
+    for row in detail.nodes:
+        row.curve_lost = bool(row.cached_from) and row.cached_from not in kept
     detail.artifacts = [
         ArtifactRow.model_validate(a, from_attributes=True) for a in artifacts
     ]
@@ -824,8 +837,8 @@ def _drop_runs(session: Session, chosen: Any) -> None:
     Two things this costs, both deliberate. ``RunNode.outputs`` *is* the stage
     cache, so a later run loses hits these would have served. And a node
     restored from one of them points here through ``cached_from`` — ``_series``
-    already reads a missing source as an empty curve, which is what ``NO_CURVE``
-    explains on the screen. The artifact bytes need no help: ``sweep_artifacts``
+    already reads a missing source as an empty curve, and ``read_run`` marks
+    such a node ``curve_lost``. The artifact bytes need no help: ``sweep_artifacts``
     keeps whatever a ``run_artifact`` row or a live message still names, so
     dropping the rows is enough and the hourly sweep reclaims the blobs.
     """

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: