🐛 fix(metrics): commit snapshots in one transaction

[gaborbernat]

Peryx committed lifetime totals before daily buckets. A storage failure between those commits produced snapshots from different event frontiers, which a restart loaded as one state.

The metrics store accepts each pair as one checkpoint. Peryx commits both keys in one redb write transaction and loads them through one redb snapshot read. The aggregator keeps a failed checkpoint pending for the next flush.

Closes #1403

[codspeed]

Merging this PR will not alter performance

✅ 23 untouched benchmarks
⏩ 139 skipped benchmarks¹

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_{Comparing fix/metrics-checkpoint-1403 (949adf3) with main (60698d6)}

Footnotes

1. 139 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩