Fix overview unselected metrics to use data-adaptive window

[unjica]

Summary

Fixes confusing ~840-day metrics window when Overview time range is left unselected.

Recommendation (options evaluated):

Option	Verdict
1. Fixed preset (30d) for unselected metrics	Good fallback; implemented as empty-data default
2. Data-adaptive window (last N events/errors + min/max caps)	Implemented — volume-aware without arbitrary calendar length
3. Align metrics with visible list page	Too narrow; rejected
4. Chart buckets over data span	Partially addressed — charts now share the metrics window when unselected (120-bucket cap unchanged)

Before: Unselected range set bucket: week and effectiveIngestRateDurationMs = 120 × 7 days ≈ 840 days for headline metrics, compare deltas, and ingest-rate denominators — regardless of whether the project had 100k events/day or 1/day.

After: With no time filter:

• Lists stay all-time (latest rows, any age)
• Metrics, charts, compare use a data-adaptive window: time span covered by the most recent 10k events + 10k error occurrences, clamped to 7–90 days, 30-day fallback when empty
• API returns metricsSince, metricsUntil, metricsDurationMs; dashboard uses the server-computed duration for ingest-rate display

Changes

• New overview-metrics-window.ts with resolveUnselectedMetricsWindow() and unit tests
• Overview API route uses adaptive window + aligned chart/session bucket when unselected
• Compare logic unchanged structurally — still uses resolveCompareWindow(metricsWindow.durationMs, …) but with the resolved adaptive duration
• Dashboard docs updated (/docs/dashboard)

Test plan

• pnpm lint && pnpm test && pnpm build
• Bugbot review (no findings)
• Manual: Overview with no time filter on high-volume project — metrics/chart span ≈ recent activity, not 840d
• Manual: Overview with no time filter on sparse project — window at least 7d, at most 90d
• Manual: Select explicit 7d/30d preset — behavior unchanged
• Manual: Error/event lists with no filter still show all-time paginated rows

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