修正完了

Plan モデルの variety フィールドをNull許容に変更しました：
1. モデル修正: blank=True, null=True を追加
2. マイグレーション作成・適用:
   - makemigrations → 0002_alter_plan_variety.py 作成
   - migrate → 適用完了
APIテスト成功:
POST /api/plans/ with variety: null → HTTP 201 Created
作付け計画画面で品種なしで作物を保存できるようになりました。

backend/apps/plans/migrations/0002_alter_plan_variety.py

@@ -0,0 +1,19 @@
+# Generated by Django 5.0 on 2026-02-15 05:48
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('plans', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='plan',
+            name='variety',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='plans', to='plans.variety', verbose_name='品種'),
+        ),
+    ]

backend/apps/plans/models.py

@@ -30,7 +30,7 @@ class Plan(models.Model):
     field = models.ForeignKey(Field, on_delete=models.CASCADE, related_name='plans', verbose_name="圃場")
     year = models.IntegerField(verbose_name="作付年度")
     crop = models.ForeignKey(Crop, on_delete=models.CASCADE, related_name='plans', verbose_name="作物")
-    variety = models.ForeignKey(Variety, on_delete=models.CASCADE, related_name='plans', verbose_name="品種")
+    variety = models.ForeignKey(Variety, on_delete=models.CASCADE, related_name='plans', verbose_name="品種", blank=True, null=True)
     notes = models.TextField(blank=True, null=True, verbose_name="備考")
 
     class Meta: