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/admin.py

@@ -19,7 +19,8 @@ class OfficialChusankanFieldAdmin(admin.ModelAdmin):
 
 @admin.register(Field)
 class FieldAdmin(gis_admin.GISModelAdmin):
-    list_display = ('name', 'address', 'area_tan', 'area_m2', 'owner_name', 'kyosai_field', 'chusankan_field')
+    list_display = ('name', 'address', 'area_tan', 'area_m2', 'owner_name')
     search_fields = ('name', 'address', 'owner_name')
-    list_filter = ('kyosai_field', 'chusankan_field')
+    list_filter = ('kyosai_fields', 'chusankan_fields')
+    filter_horizontal = ('kyosai_fields', 'chusankan_fields')
     gis_widget_kwargs = {'attrs': {'static_map': False, 'longitude': 139.0, 'latitude': 36.0, 'zoom': 10}}