4 tasks: mark a wrapper-level 404 as not_found, expose it as
garmin.ErrNotFound in the Go client, add a workout_not_found_at
column + store query updates, then have fillPendingWorkouts stop
retrying a confirmed-404 workout instead of retrying forever.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
