[Bug]: MAX_CONTRIBUTION_BONUS fallback is 5x upstream (25 vs 5)

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Found in the 2026-06-17 scoring-sync audit. Silent 5x inflation on fetch failure.

Problem

Local fallback MAX_CONTRIBUTION_BONUS = 25 (src/scoring/model.ts:14), duplicated as the constant() fallback in src/scoring/preview.ts:961 — but upstream is 5. On every successful hourly fetch the parsed 5 wins, so live scoring is correct; but if a fetch ever fails, the contribution bonus is silently 5x too high → wrong scores/previews.

Deliverable

• Set the fallback to 5 (match upstream) in model.ts:14 and preview.ts:961
• Add a regression test that a failed fetch uses 5, not 25

Outcome

Fetch-failure fallback no longer inflates scores 5x. Milestone M0.