fix(sync): stop retrying a workout Garmin confirms is gone

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.

backend/internal/sync/service.go

@@ -7,6 +7,7 @@ package sync
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log"
 	"sync"
@@ -363,7 +364,18 @@ func (s *Service) fillPendingWorkouts(ctx context.Context, limit int) error {
 			}
 		}
 		if err := s.fillActivityWorkout(ctx, a, profile); err != nil {
-			log.Printf("sync: fill workout for activity %d failed, will retry next sync: %v", a.GarminActivityID, err)
+			if errors.Is(err, garmin.ErrNotFound) {
+				// A definitive 404 (the workout was deleted on Garmin's side
+				// after being linked to this activity) will never succeed on
+				// retry -- mark it so ActivitiesMissingWorkout stops
+				// surfacing it, instead of retrying forever.
+				log.Printf("sync: workout for activity %d not found on Garmin, marking as such (will not retry): %v", a.GarminActivityID, err)
+				if serr := s.db.SetActivityWorkoutNotFound(ctx, s.userID, a.ID); serr != nil {
+					return fmt.Errorf("mark activity %d workout not found: %w", a.GarminActivityID, serr)
+				}
+			} else {
+				log.Printf("sync: fill workout for activity %d failed, will retry next sync: %v", a.GarminActivityID, err)
+			}
 		}
 		s.setProgress(PhaseWorkouts, i+1, len(pending))
 	}