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/materials/migrations/0004_fix_spreading_item_on_delete.py

@@ -0,0 +1,20 @@
+# Generated by Django 5.0 on 2026-03-17 10:44
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('fertilizer', '0008_spreadingsession_fertilizationentry_actual_bags_and_more'),
+        ('materials', '0003_stocktransaction_spreading_item_and_more'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='stocktransaction',
+            name='spreading_item',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='stock_transactions', to='fertilizer.spreadingsessionitem', verbose_name='散布実績明細'),
+        ),
+    ]