ManyToMany 関係への変更が完了しました。

変更内容
models.py:
- kyosai_field (ForeignKey) → kyosai_fields (ManyToManyField)
- chusankan_field (ForeignKey) → chusankan_fields (ManyToManyField)
admin.py:
- list_display から削除
- list_filter を _fields に変更
- filter_horizontal を追加（複数選択用UI）
マイグレーション: ✅ 完了
管理画面 (http://localhost:8000/admin/fields/field/) で、複数の共済区画・中山間区画を選択できます。

backend/apps/fields/migrations/0003_remove_field_chusankan_field_and_more.py

@@ -0,0 +1,31 @@
+# Generated by Django 5.0 on 2026-02-15 02:56
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('fields', '0002_field_raw_chusankan_id_field_raw_kyosai_k_num_and_more'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='field',
+            name='chusankan_field',
+        ),
+        migrations.RemoveField(
+            model_name='field',
+            name='kyosai_field',
+        ),
+        migrations.AddField(
+            model_name='field',
+            name='chusankan_fields',
+            field=models.ManyToManyField(blank=True, related_name='fields', to='fields.officialchusankanfield', verbose_name='関連中山間マスタ'),
+        ),
+        migrations.AddField(
+            model_name='field',
+            name='kyosai_fields',
+            field=models.ManyToManyField(blank=True, related_name='fields', to='fields.officialkyosaifield', verbose_name='関連共済マスタ'),
+        ),
+    ]