GarminTokenStore is renamed GarminTokenStoreRoot to reflect that it now
roots one subdirectory per user rather than a single session cache path.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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.