NerdCorner VAM: filter short climbs, opacity-encode confidence, add climbing time to tooltip

- Exclude per-activity VAM contributions where climbing_time_s < 10 min; short
  punchy efforts don't represent aerobic fitness and were skewing monthly averages
- Store climbing_time_s alongside climbing_vam_mh in metrics, detail JSON, and
  summary JSON so the frontend has the data to reason about confidence
- Accumulate total climbing time per period; opacity scales from 0.25 (10 min,
  minimum threshold) to 1.0 (≥ 1 h) so thin-evidence months read as faint dots
- Render VAM as dots only (no lines) since each period is an independent average,
  not a cumulative — lines implied continuity that isn't there
- Tooltip now shows "1060 m/h · 38 min climbing"

site/src/lib/types.ts

@@ -67,6 +67,7 @@ export interface ActivitySummary {
   avg_power_w: number | null;
   mmp: MmpCurve | null;
   climbing_vam_mh?: number | null;
+  climbing_time_s?: number | null;
   source: string | null;
   privacy: Privacy;
   detail_url: string | null;