施肥計画の計算設定を保存・復元し、未入力圃場のみ計算オプションを追加

- FertilizationPlanにcalc_settings JSONFieldを追加（migration 0004）
- 編集画面を開くと前回の計算方式・パラメータが復元される
- 「未入力圃場のみ」チェックで既存値を保持したまま新規圃場だけ計算可能

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

backend/apps/fertilizer/migrations/0004_fertilizationplan_calc_settings.py

@@ -0,0 +1,16 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('fertilizer', '0003_distributionplan_distributiongroup_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='fertilizationplan',
+            name='calc_settings',
+            field=models.JSONField(blank=True, default=list, verbose_name='計算設定'),
+        ),
+    ]