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/admin.py

@@ -1,5 +1,5 @@
 from django.contrib import admin
-from .models import Fertilizer, FertilizationPlan, FertilizationEntry
+from .models import Fertilizer, FertilizationPlan, FertilizationEntry, DistributionPlan, DistributionGroup, DistributionGroupField
 
 
 @admin.register(Fertilizer)
@@ -17,3 +17,27 @@ class FertilizationPlanAdmin(admin.ModelAdmin):
     list_display = ['name', 'year', 'variety']
     list_filter = ['year']
     inlines = [FertilizationEntryInline]
+
+
+class DistributionGroupFieldInline(admin.TabularInline):
+    model = DistributionGroupField
+    extra = 0
+    readonly_fields = ['distribution_plan']
+
+
+class DistributionGroupInline(admin.TabularInline):
+    model = DistributionGroup
+    extra = 0
+
+
+@admin.register(DistributionPlan)
+class DistributionPlanAdmin(admin.ModelAdmin):
+    list_display = ['name', 'fertilization_plan', 'created_at']
+    list_filter = ['fertilization_plan__year']
+    inlines = [DistributionGroupInline]
+
+
+@admin.register(DistributionGroup)
+class DistributionGroupAdmin(admin.ModelAdmin):
+    list_display = ['name', 'distribution_plan', 'order']
+    inlines = [DistributionGroupFieldInline]