fix(hl-history): raise staleness threshold to 2h

[Flotapponnier]

Summary

• readCohortSnapshot for the hl-history blob used the default 10-min max age
• History data is daily-stepped (changes once per day), so 10 min is too tight
• When stale, Vercel triggers fetchHlHistoryFresh() which does ~200 Prom range queries — expensive and slow
• Raises the threshold to 2h so the blob is reused as long as it's recent enough

Effect

• Fewer unnecessary full history rebuilds from Vercel
• Reduces flicker where history = null during the rebuild window → placeholder shown
• The HlBuilderNoHistory fallback (already on dev) handles the rare case where history is genuinely absent