Persist official climb attempts with climbs pages

Record one attempt row per (ride, climb, occurrence) when a ride detail
loads, snapshotting distance, elevation gain, slope, cotacol and attempt
timestamps; a climbs-backfill CLI command seeds all existing rides.

New /climbs list page aggregates attempts (medians computed in Go, best
time as min duration) with sortable columns, and /climbs/{id} shows the
median scalar stats, a median normalized elevation profile (50 steps
over each attempt's own length), the full-precision recorded route of
the median-distance attempt on an OSM map, and every attempt with time
at second precision.

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