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_test.go

@@ -902,6 +902,68 @@ func TestFillPendingDetails_OneWorkoutFetchFailureDoesNotAbortTheWorkoutsPass(t
 	}
 }
 
+func TestFillPendingDetails_NotFoundWorkoutStopsBeingRetried(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+	userID := provisionTestUser(t, db)
+
+	const missingWorkoutID = 300
+	missingPtr := int64(missingWorkoutID)
+	m := &mock.Client{
+		Activities: []garmin.Activity{
+			{ActivityID: 1, ActivityType: garmin.ActivityType{TypeKey: "running"}, WorkoutID: &missingPtr,
+				StartTimeGMT: "2026-07-01 06:00:00", Distance: 5000, Duration: 1500},
+		},
+		Splits: map[int64]garmin.ActivitySplits{
+			1: {ActivityID: 1, Laps: []garmin.Lap{{LapIndex: 1, IntensityType: "ACTIVE"}}},
+		},
+		Details:        map[int64]garmin.ActivityDetails{1: {ActivityID: 1}},
+		Workouts:       map[int64]garmin.Workout{},
+		WorkoutErrByID: map[int64]error{missingWorkoutID: fmt.Errorf("API Error 404: %w", garmin.ErrNotFound)},
+	}
+	svc := NewService(m, db, userID, Config{InterCallDelay: time.Millisecond},
+		fixedNow(time.Date(2026, 7, 11, 0, 0, 0, 0, time.UTC)))
+
+	if _, err := svc.backfillCore(ctx); err != nil {
+		t.Fatalf("backfillCore: %v", err)
+	}
+	if err := svc.FillPendingDetails(ctx, 10); err != nil {
+		t.Fatalf("FillPendingDetails should not return an error for a confirmed-404 workout: %v", err)
+	}
+
+	remaining, err := db.CountActivitiesMissingWorkout(ctx, userID)
+	if err != nil {
+		t.Fatalf("CountActivitiesMissingWorkout: %v", err)
+	}
+	if remaining != 0 {
+		t.Fatalf("CountActivitiesMissingWorkout = %d, want 0 (confirmed-404 activity must not be retried)", remaining)
+	}
+
+	activities, err := db.ListActivities(ctx, userID, store.ActivityFilter{})
+	if err != nil {
+		t.Fatalf("ListActivities: %v", err)
+	}
+	if len(activities) != 1 {
+		t.Fatalf("ListActivities returned %d activities, want 1", len(activities))
+	}
+	got, _, err := db.GetActivity(ctx, userID, activities[0].ID)
+	if err != nil {
+		t.Fatalf("GetActivity: %v", err)
+	}
+	if got.WorkoutNotFoundAt == nil {
+		t.Error("WorkoutNotFoundAt is nil, want it set")
+	}
+	if got.WorkoutRawJSON != nil {
+		t.Error("WorkoutRawJSON should stay nil -- not-found is recorded separately, not faked")
+	}
+
+	// A second FillPendingDetails call must not attempt this workout again
+	// (it's no longer in ActivitiesMissingWorkout's result set at all).
+	if err := svc.FillPendingDetails(ctx, 10); err != nil {
+		t.Fatalf("second FillPendingDetails: %v", err)
+	}
+}
+
 func TestService_TwoUsersSyncIndependently(t *testing.T) {
 	db := openTestDB(t)
 	ctx := context.Background()