ci: Capture actual task durations under turbo 2.x summary schema

[shortstacked]

Summary

• .github/scripts/send-build-stats.mjs was reading task.execution.durationMs and summary.durationMs — neither exist in turbo 2.x summaries
• Every row written to qa_performance_metrics therefore had value = 0, silently breaking all duration-based queries against the table
• Fix derives durations from execution.startTime / endTime for misses, and uses cache.timeSaved for hits (the duration of the run that was avoided — more useful than the ~ms cache-restore window)

Linear: https://linear.app/n8n/issue/DEVP-173

Root cause

Turbo 2.x summary schema (confirmed against an actual .turbo/runs/*.json from 2.9.4):

"execution": {
  "startTime": 1779132055452,
  "endTime": 1779132058010,
  "exitCode": 0
}

No durationMs field. The ?? 0 fallback in the script null-coalesced every row to 0. Counts (hit/miss) survived because they come from task.cache.status, but no timing data ever reached BigQuery under this turbo version.

Bonus capture: cache.timeSaved

Cache hits include cache.timeSaved — the duration of the task whose execution turbo skipped. Far more useful for analytics than the cache-restore time (typically <5 ms). Now captured as the hit-side duration.

Smoke test

Patched logic verified locally against a real summary file (build run, 59 tasks):

• 32/32 cache hits → non-zero duration (avg 2.77 s, from cache.timeSaved)
• 27/27 cache misses → non-zero duration (avg 4.38 s, from endTime − startTime)
• Total run duration: 63.66 s (was 0 before)

Impact

• Going-forward qa_performance_metrics rows have correct durations
• Unblocks timing columns in CI cache dashboard (avg_miss_s, p95_miss_s, avg_hit_s, total_miss_hours)
• Required for measuring DEVP-171 (affected-only filter) impact — without this fix, hit/miss rate changes would be visible but the cost-saved signal wouldn't be
• Historical 0-value rows stay 0 (no backfill — turbo summaries aren't archived)

Expect MAX(value) > 0 for both hit and miss rows.

Test plan

• I have seen this code, I have run this code, and I take responsibility for this code.
• Merge and watch a CI run land
• Run the verification query 1 hour after merge — confirm non-zero values
• Open qa-ci-cache-hit-dashboard.md queries in Hex against the fresh data — timing columns should populate

🤖 Generated with Claude Code

[n8n-assistant]

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