Keep a failed node's traceback on the run

The worker already sent it and the log panel already got it; the failure
outcome kept the one-line error and the node's stdout and dropped the
rest, so reading a failure back meant reproducing it under `run --local`.
It rides in the node's logs now — no schema change, and the API row, the
run detail page and `RunHandle.failures` carry it as they are.

`_record_node` keeps the tail of the log cap rather than the head, so a
chatty node cannot push the traceback past it, and `fluksio run` prints
what each node said when a run does not end ok.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TXQv6KNyyvY7Z1etYTUUAd

backend/tests/test_cli.py

@@ -282,6 +282,53 @@ def test_ctrl_c_while_waiting_cancels_the_run(monkeypatch) -> None:
     assert cancelled == ["run-1"]
 
 
+def test_a_failed_run_prints_what_each_node_said(monkeypatch, capsys) -> None:
+    """Otherwise the traceback is stored and nothing at a terminal shows it."""
+    from contextlib import contextmanager
+
+    from fluksio.cli import _parser
+    from fluksio.sdk import cli
+
+    class FakeHandle:
+        id = "run-1"
+        status = "error"
+        result: dict[str, object] = {}
+        failures = [
+            {
+                "node": "study.train",
+                "error": "ValueError: no convergence",
+                "logs": 'Traceback (most recent call last):\n  File "<node>"\n',
+            }
+        ]
+
+        def wait(self, timeout: float = 0.0) -> "FakeHandle":
+            return self
+
+    class FakeClient:
+        def get_flow(self, name: str) -> dict[str, object]:
+            return {"definition": {"inputs": []}}
+
+        def submit(self, flow, params, seed=None, no_cache=False, cause="sdk"):
+            return FakeHandle()
+
+        def run(self, run_id: str) -> dict[str, object]:
+            return {"nodes": []}
+
+    @contextmanager
+    def fake_engine():
+        yield FakeClient()
+
+    monkeypatch.setattr(cli, "_engine_client", fake_engine)
+
+    args = _parser().parse_args(["run", "train", "--local", "--no-sync"])
+    assert cli.cmd_run(args, []) == 1
+
+    printed = capsys.readouterr().out
+    assert "study.train" in printed
+    assert "ValueError: no convergence" in printed
+    assert "Traceback (most recent call last):" in printed
+
+
 def test_an_artifact_input_may_be_named_rather_than_pasted() -> None:
     """The engine resolves either spelling; the CLI just stops mangling them."""
     import json