分配計画機能を実装

施肥計画の圃場を配置場所単位でグループ化し、グループ×肥料の集計表を
表示・PDF出力できる機能を追加。

- Backend: DistributionPlan/Group/GroupField モデル (migration 0003)
- API: GET/POST/PUT/DELETE/PDF (/api/fertilizer/distribution/)
- Frontend: 一覧・新規作成・編集画面 (/distribution)
- Navbar に分配計画メニューを追加
- 集計プレビューはクライアントサイド計算（API不要）

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

backend/apps/fertilizer/migrations/0003_distributionplan_distributiongroup_and_more.py

@@ -0,0 +1,60 @@
+# Generated by Django 5.0 on 2026-03-01 15:46
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('fertilizer', '0002_alter_fertilizationentry_fertilizer'),
+        ('fields', '0006_e1c_chusankan_17_fields'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='DistributionPlan',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=200, verbose_name='計画名')),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('updated_at', models.DateTimeField(auto_now=True)),
+                ('fertilization_plan', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='distribution_plans', to='fertilizer.fertilizationplan', verbose_name='施肥計画')),
+            ],
+            options={
+                'verbose_name': '分配計画',
+                'verbose_name_plural': '分配計画',
+                'ordering': ['-fertilization_plan__year', 'name'],
+            },
+        ),
+        migrations.CreateModel(
+            name='DistributionGroup',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=100, verbose_name='グループ名')),
+                ('order', models.PositiveIntegerField(default=0, verbose_name='表示順')),
+                ('distribution_plan', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='groups', to='fertilizer.distributionplan', verbose_name='分配計画')),
+            ],
+            options={
+                'verbose_name': '分配グループ',
+                'verbose_name_plural': '分配グループ',
+                'ordering': ['order', 'id'],
+                'unique_together': {('distribution_plan', 'name')},
+            },
+        ),
+        migrations.CreateModel(
+            name='DistributionGroupField',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('field', models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, to='fields.field', verbose_name='圃場')),
+                ('group', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='field_assignments', to='fertilizer.distributiongroup', verbose_name='グループ')),
+                ('distribution_plan', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='fertilizer.distributionplan', verbose_name='分配計画')),
+            ],
+            options={
+                'verbose_name': 'グループ圃場割り当て',
+                'verbose_name_plural': 'グループ圃場割り当て',
+                'ordering': ['field__display_order', 'field__id'],
+                'unique_together': {('distribution_plan', 'field')},
+            },
+        ),
+    ]