SMARTRUN_BACKFILL_HORIZON_DAYS was a server-startup-only env var with no UI,
defaulting to 3 years -- so editing the unrelated "Rolling window" profile
field (for classification, not sync) had no effect on how far back Sync Now
reached. Backfill horizon is now Profile.BackfillHorizonDays, read fresh on
every Backfill call, with its own field on the Profile page.
Update workout kinds by fixed name (via ListWorkoutKinds/UpdateWorkoutKind)
instead of creating new ones, since the taxonomy migration now seeds
Easy Run/Tempo/Interval by unique name. Set profile.MaxHeartRate so
avg_hr_pct_max is computed during classification, and drop the removed
appsync.Config.MaxHR field.
