散布実績レビュー修正: バグ修正・仕様適合・デッドコード削除

- 候補API: 運搬済みフィルタ(date IS NOT NULL)を追加。
  delivery_plan_id指定時は全明細表示、年度全体時のみ日付フィルタ適用
- StockTransaction.spreading_item: CASCADE→SET_NULL に修正（仕様7.3準拠）
- perform_destroy: SET_NULL対応でUSEを明示削除してからsession削除
- ConfirmSpreadingModal.tsx: 未使用のため削除
- FertilizerEditPage.tsx: 旧散布確定関連デッドコード全除去
  (isConfirmed/confirmedAt state, handleUnconfirm, 確定取消ボタン, 確定済みバナー)
- services.py: 未使用のto_decimal_or_zero削除

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/apps/fertilizer/views.py

@@ -381,8 +381,10 @@ class SpreadingSessionViewSet(viewsets.ModelViewSet):
         return SpreadingSessionSerializer
 
     def perform_destroy(self, instance):
+        from apps.materials.models import StockTransaction
         year = instance.year
         affected_pairs = {(item.field_id, item.fertilizer_id) for item in instance.items.all()}
+        StockTransaction.objects.filter(spreading_item__session=instance).delete()
         instance.delete()
         sync_actual_bags_for_pairs(year, affected_pairs)
 
@@ -485,6 +487,8 @@ class SpreadingCandidatesView(APIView):
         delivery_queryset = DeliveryTripItem.objects.filter(trip__delivery_plan__year=year)
         if delivery_plan_id:
             delivery_queryset = delivery_queryset.filter(trip__delivery_plan_id=delivery_plan_id)
+        else:
+            delivery_queryset = delivery_queryset.filter(trip__date__isnull=False)
         delivery_rows = delivery_queryset.values(
             'field_id',
             'field__name',