feat(dashboard): peak-time timestamps on daily extremes

[kanfei-code-agent]

Summary

• Each daily extreme on the dashboard now carries an at ISO-8601 UTC timestamp; the dashboard renders the time underneath the value so a user can tell whether "Peak 38 mph" happened at 3 AM in a storm or five minutes ago.
• Covers wind peak, rain rate peak, and the H/L pair on every temperature, humidity, and barometer tile. Solar/UV has no daily extreme today and is out of scope.
• Mobile CompactCard rendering deliberately unchanged — secondary line already shows H/L values.

Implementation

• backend/app/services/daily_extremes.py: keep the single aggregate query for the 12 extremum values; add one MIN(timestamp) follow-up per non-null extreme, filtered column == raw_value AND timestamp >= midnight_utc. Earliest occurrence wins for ties. _val() now emits {"value", "unit", "at"}; naive SQLite UTC datetimes are re-stamped to UTC and serialised with a Z suffix so new Date(iso) on the frontend parses correctly.
• frontend/src/api/types.ts: new ExtremeReading type ({ value, unit, at: string | null }); DailyExtremes fields swap from ValueWithUnit | null to ExtremeReading | null.
• TileRenderer plumbs highAt / lowAt / peakAt / peakRateAt into each gauge. Each gauge calls the existing formatTimestamp helper to render either "at HH:MM AM/PM" (wind peak, rain peak) or "H 2:45 PM · L 11:30 PM" (Temp / Humidity / Barometer).

Review

• qwen3:30b: APPROVE
• Codex (final): APPROVE — non-blocking notes: per-column index could speed the _at scans (timestamp index alone bounds them to today's ~8.6K rows, ~12 scans per request), and tests cover tied maxima but not tied minima symmetrically.

Test plan

• cd backend && python -m pytest ../tests/backend/ -q — 426 pass (was 421; +5 from new test_daily_extremes.py)
• cd frontend && npx tsc --noEmit — clean
• cd frontend && npm run build — clean
• py_compile on changed Python files — clean
• Manual: load the dashboard, confirm Wind tile shows "Peak X mph / at HH:MM" and Temperature/Humidity/Barometer tiles show "H .. · L .." times. Reset DB → confirm no timestamp lines render before the first reading.

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