Build the Review Queue chart's actual trace from per-second samples, not lap averages

A lap can span many minutes, so plotting one flat value per lap hid real
within-lap variation -- a single-lap hill repeat rendered as a dead-flat
line despite the pace/HR swinging throughout it. The actual pace/HR trace
now comes from activity_samples (per-second telemetry) when available,
looking up each sample's enclosing lap only for the target band/phase
color. Falls back to the previous lap-stepped rendering when an activity
has no samples.

backend/internal/api/api_test.go

@@ -170,6 +170,12 @@ func TestReviewQueueResolve(t *testing.T) {
 	}); err != nil {
 		t.Fatalf("ReplaceLaps: %v", err)
 	}
+	hr := 150.0
+	if err := db.ReplaceActivitySamples(ctx, activityID, []store.Sample{
+		{ElapsedSeconds: 0, HeartRate: &hr},
+	}); err != nil {
+		t.Fatalf("ReplaceActivitySamples: %v", err)
+	}
 
 	router := s.Router()
 	rec := doJSON(t, router, http.MethodGet, "/api/review-queue/", nil)
@@ -185,6 +191,10 @@ func TestReviewQueueResolve(t *testing.T) {
 	if len(laps) != 1 {
 		t.Fatalf("expected 1 lap in review queue item, got %d: %s", len(laps), rec.Body.String())
 	}
+	samples, _ := queue[0]["samples"].([]any)
+	if len(samples) != 1 {
+		t.Fatalf("expected 1 sample in review queue item, got %d: %s", len(samples), rec.Body.String())
+	}
 
 	rec = doJSON(t, router, http.MethodPost, "/api/review-queue/"+itoa(activityID)+"/resolve", map[string]any{"workout_kind_id": kindID})
 	if rec.Code != http.StatusOK {