feat(review): replace flat MINUTES_SAVED_PER_PR with the per-PR effort estimate in public-stats ROI

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Part of #1955. The review-time-saved ROI currently credits a flat 20 min/PR (MINUTES_SAVED_PER_PR, src/review/public-stats.ts:28,295). Replace it with the per-PR estimate from the estimator so the ROI figure is credible. Depends on the estimator module (sibling bounty). Keep the aggregate self-host-only per the epic.

Deliverables

• Replace the flat reviewed * MINUTES_SAVED_PER_PR sum (src/review/public-stats.ts:295) with a sum of per-PR estimated minutes; retain MINUTES_SAVED_PER_PR only as a documented fallback for PRs lacking a stored estimate
• Persist or thread the per-PR estimated minutes so the aggregate can sum real values (reuse an existing stored field if present; a migration must be single-file + type-regenerated if needed)
• Keep the byte-identical zero-review path (minutesSaved: 0, src/review/public-stats.ts:176)
• Unit tests: mixed-effort PRs sum correctly, fallback used when estimate missing, zero-reviewed ⇒ 0; 97% branch (test the fallback nullish arm)

References

• src/review/public-stats.ts:28
• src/review/public-stats.ts:295
• src/review/public-stats.ts:176
• #1955

Part of #1955.

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size: S · gittensor:feature

[linear-code]

JSO-2187