fillPendingWorkouts previously treated every get_workout_by_id
failure identically -- log it, leave workout_raw_json null, retry
next sync -- which is correct for a transient failure but means a
definitive 404 (the workout deleted on Garmin's side after being
linked to an activity) got silently retried forever, with the only
symptom being an unexplained, permanent "1 more workout pending"
nudge. Now checks errors.Is(err, garmin.ErrNotFound) and, only for
that case, calls SetActivityWorkoutNotFound instead of retrying.