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/models.py

@@ -67,3 +67,64 @@ class FertilizationEntry(models.Model):
 
     def __str__(self):
         return f"{self.plan} / {self.field} / {self.fertilizer}: {self.bags}袋"
+
+
+class DistributionPlan(models.Model):
+    """分配計画：施肥計画の圃場をカスタムグループに割り当て、配置場所単位で集計する"""
+    fertilization_plan = models.ForeignKey(
+        FertilizationPlan, on_delete=models.CASCADE,
+        related_name='distribution_plans', verbose_name='施肥計画'
+    )
+    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)
+
+    class Meta:
+        verbose_name = '分配計画'
+        verbose_name_plural = '分配計画'
+        ordering = ['-fertilization_plan__year', 'name']
+
+    def __str__(self):
+        return f"{self.fertilization_plan.year} {self.name}"
+
+
+class DistributionGroup(models.Model):
+    """分配グループ：ある場所にまとめて置く圃場のグループ"""
+    distribution_plan = models.ForeignKey(
+        DistributionPlan, on_delete=models.CASCADE,
+        related_name='groups', verbose_name='分配計画'
+    )
+    name = models.CharField(max_length=100, verbose_name='グループ名')
+    order = models.PositiveIntegerField(default=0, verbose_name='表示順')
+
+    class Meta:
+        verbose_name = '分配グループ'
+        verbose_name_plural = '分配グループ'
+        unique_together = [['distribution_plan', 'name']]
+        ordering = ['order', 'id']
+
+    def __str__(self):
+        return f"{self.distribution_plan} / {self.name}"
+
+
+class DistributionGroupField(models.Model):
+    """圃場のグループへの割り当て（1圃場=1グループ/1分配計画）"""
+    distribution_plan = models.ForeignKey(
+        DistributionPlan, on_delete=models.CASCADE, verbose_name='分配計画'
+    )
+    group = models.ForeignKey(
+        DistributionGroup, on_delete=models.CASCADE,
+        related_name='field_assignments', verbose_name='グループ'
+    )
+    field = models.ForeignKey(
+        'fields.Field', on_delete=models.PROTECT, verbose_name='圃場'
+    )
+
+    class Meta:
+        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'
+        unique_together = [['distribution_plan', 'field']]
+        ordering = ['field__display_order', 'field__id']
+
+    def __str__(self):
+        return f"{self.group.name} / {self.field.name}"