VAM: drop duration curve, show avg climbing VAM in Nerd Corner

Remove the per-duration VAM curve everywhere (metrics, summaries, detail
JSON, athlete.json, VamChart.svelte, AthleteView VAM tab). Keep only
climbing_vam_mh per activity. Add it to activity summaries so NerdCorner
can plot average climbing VAM per week/month year-over-year alongside
distance/elevation/time. Add --backfill-vam-summary flag to copy the
field from existing detail JSONs into index.json without re-extracting.

site/src/lib/types.ts

@@ -36,17 +36,10 @@ export interface BestClimb {
   title: string;
 }
 
-export interface AthleteVamCurve {
-  all_time:  MmpCurve | null;
-  last_365d: MmpCurve | null;
-  last_90d:  MmpCurve | null;
-}
-
 export interface AthleteJson {
   bas_version: string;
   generated_at: string;
   power_curve: AthletePowerCurve;
-  vam_curve?: AthleteVamCurve | null;
   records?: Record<string, Record<string, EffortRecord | ValueRecord>>;
   best_climbs?: BestClimb[];
   max_hr?: number;
@@ -73,7 +66,7 @@ export interface ActivitySummary {
   avg_cadence_rpm: number | null;
   avg_power_w: number | null;
   mmp: MmpCurve | null;
-  vam_curve?: MmpCurve | null;
+  climbing_vam_mh?: number | null;
   source: string | null;
   privacy: Privacy;
   detail_url: string | null;
@@ -130,7 +123,6 @@ export interface ActivityDetail extends Omit<ActivitySummary, 'detail_url' | 'tr
   /** URL to fetch the timeseries — present for server-extracted activities. */
   timeseries_url?: string | null;
   mmp: MmpCurve | null;
-  climbing_vam_mh?: number | null;
   strava_id: string | null;
   duplicate_of: string | null;
   source_file?: string | null;