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/serializers.py

@@ -1,5 +1,5 @@
 from rest_framework import serializers
-from .models import Fertilizer, FertilizationPlan, FertilizationEntry
+from .models import Fertilizer, FertilizationPlan, FertilizationEntry, DistributionPlan, DistributionGroup, DistributionGroupField
 
 
 class FertilizerSerializer(serializers.ModelSerializer):
@@ -79,3 +79,140 @@ class FertilizationPlanWriteSerializer(serializers.ModelSerializer):
                 fertilizer_id=entry['fertilizer_id'],
                 bags=entry['bags'],
             )
+
+
+# ─── 分配計画 ────────────────────────────────────────────────────────────
+
+class DistributionGroupFieldSerializer(serializers.ModelSerializer):
+    id = serializers.IntegerField(source='field.id', read_only=True)
+    name = serializers.CharField(source='field.name', read_only=True)
+    area_tan = serializers.DecimalField(
+        source='field.area_tan', max_digits=6, decimal_places=4, read_only=True
+    )
+
+    class Meta:
+        model = DistributionGroupField
+        fields = ['id', 'name', 'area_tan']
+
+
+class DistributionGroupReadSerializer(serializers.ModelSerializer):
+    fields = DistributionGroupFieldSerializer(source='field_assignments', many=True, read_only=True)
+
+    class Meta:
+        model = DistributionGroup
+        fields = ['id', 'name', 'order', 'fields']
+
+
+class FertilizationPlanForDistributionSerializer(serializers.ModelSerializer):
+    """分配計画詳細に埋め込む施肥計画情報（肥料一覧・entries 含む）"""
+    variety_name = serializers.SerializerMethodField()
+    crop_name = serializers.SerializerMethodField()
+    fertilizers = serializers.SerializerMethodField()
+    entries = serializers.SerializerMethodField()
+
+    class Meta:
+        model = FertilizationPlan
+        fields = ['id', 'name', 'year', 'variety_name', 'crop_name', 'fertilizers', 'entries']
+
+    def get_variety_name(self, obj):
+        return obj.variety.name
+
+    def get_crop_name(self, obj):
+        return obj.variety.crop.name
+
+    def get_fertilizers(self, obj):
+        fert_ids = obj.entries.values_list('fertilizer_id', flat=True).distinct()
+        from .models import Fertilizer as F
+        fertilizers = F.objects.filter(id__in=fert_ids).order_by('name')
+        return [{'id': f.id, 'name': f.name} for f in fertilizers]
+
+    def get_entries(self, obj):
+        return [
+            {'field': e.field_id, 'fertilizer': e.fertilizer_id, 'bags': str(e.bags)}
+            for e in obj.entries.all()
+        ]
+
+
+class DistributionPlanListSerializer(serializers.ModelSerializer):
+    fertilization_plan_id = serializers.IntegerField(source='fertilization_plan.id', read_only=True)
+    fertilization_plan_name = serializers.CharField(source='fertilization_plan.name', read_only=True)
+    year = serializers.IntegerField(source='fertilization_plan.year', read_only=True)
+    variety_name = serializers.SerializerMethodField()
+    crop_name = serializers.SerializerMethodField()
+    group_count = serializers.SerializerMethodField()
+    field_count = serializers.SerializerMethodField()
+
+    class Meta:
+        model = DistributionPlan
+        fields = [
+            'id', 'name', 'fertilization_plan_id', 'fertilization_plan_name',
+            'year', 'variety_name', 'crop_name', 'group_count', 'field_count',
+            'created_at', 'updated_at',
+        ]
+
+    def get_variety_name(self, obj):
+        return obj.fertilization_plan.variety.name
+
+    def get_crop_name(self, obj):
+        return obj.fertilization_plan.variety.crop.name
+
+    def get_group_count(self, obj):
+        return obj.groups.count()
+
+    def get_field_count(self, obj):
+        return obj.distributiongroupfield_set.count()
+
+
+class DistributionPlanReadSerializer(serializers.ModelSerializer):
+    fertilization_plan = FertilizationPlanForDistributionSerializer(read_only=True)
+    groups = DistributionGroupReadSerializer(many=True, read_only=True)
+    unassigned_fields = serializers.SerializerMethodField()
+
+    class Meta:
+        model = DistributionPlan
+        fields = ['id', 'name', 'fertilization_plan', 'groups', 'unassigned_fields', 'created_at', 'updated_at']
+
+    def get_unassigned_fields(self, obj):
+        assigned_ids = obj.distributiongroupfield_set.values_list('field_id', flat=True)
+        plan_field_ids = obj.fertilization_plan.entries.values_list('field_id', flat=True).distinct()
+        from apps.fields.models import Field as F
+        unassigned = F.objects.filter(id__in=plan_field_ids).exclude(id__in=assigned_ids).order_by('display_order', 'id')
+        return [{'id': f.id, 'name': f.name, 'area_tan': str(f.area_tan)} for f in unassigned]
+
+
+class DistributionPlanWriteSerializer(serializers.ModelSerializer):
+    fertilization_plan_id = serializers.IntegerField(write_only=True)
+    groups = serializers.ListField(child=serializers.DictField(), write_only=True, required=False)
+
+    class Meta:
+        model = DistributionPlan
+        fields = ['id', 'name', 'fertilization_plan_id', 'groups']
+
+    def create(self, validated_data):
+        groups_data = validated_data.pop('groups', [])
+        plan = DistributionPlan.objects.create(**validated_data)
+        self._save_groups(plan, groups_data)
+        return plan
+
+    def update(self, instance, validated_data):
+        groups_data = validated_data.pop('groups', None)
+        instance.name = validated_data.get('name', instance.name)
+        instance.save()
+        if groups_data is not None:
+            instance.groups.all().delete()
+            self._save_groups(instance, groups_data)
+        return instance
+
+    def _save_groups(self, plan, groups_data):
+        for g_data in groups_data:
+            group = DistributionGroup.objects.create(
+                distribution_plan=plan,
+                name=g_data['name'],
+                order=g_data.get('order', 0),
+            )
+            for field_id in g_data.get('field_ids', []):
+                DistributionGroupField.objects.create(
+                    distribution_plan=plan,
+                    group=group,
+                    field_id=field_id,
+                )