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/fluksio/flow/runs.py

@@ -1248,7 +1248,9 @@ class RunService:
             started_at=datetime.now(UTC),
             duration_ms=outcome.duration_ms,
             error=outcome.error[:ERROR_CAP],
-            logs=outcome.logs[:LOG_CAP],
+            # The tail, not the head: a failure appends its traceback, and a
+            # chatty node would otherwise push it past the cap.
+            logs=outcome.logs[-LOG_CAP:],
             cached_from=outcome.cached_from,
             # Together or not at all: a row carrying a key must be one a
             # lookup can actually restore from.