Add plan variety change tracking

backend/apps/plans/models.py

@@ -65,6 +65,51 @@ class Plan(models.Model):
         return f"{self.field.name} - {self.year} - {self.crop.name}"
 
 
+class PlanVarietyChange(models.Model):
+    field = models.ForeignKey(
+        Field,
+        on_delete=models.PROTECT,
+        related_name='plan_variety_changes',
+        verbose_name='圃場',
+    )
+    year = models.IntegerField(verbose_name='作付年度')
+    plan = models.ForeignKey(
+        Plan,
+        on_delete=models.CASCADE,
+        related_name='variety_changes',
+        verbose_name='作付け計画',
+    )
+    changed_at = models.DateTimeField(auto_now_add=True, verbose_name='変更日時')
+    old_variety = models.ForeignKey(
+        Variety,
+        on_delete=models.SET_NULL,
+        related_name='old_plan_variety_changes',
+        verbose_name='変更前品種',
+        null=True,
+        blank=True,
+    )
+    new_variety = models.ForeignKey(
+        Variety,
+        on_delete=models.SET_NULL,
+        related_name='new_plan_variety_changes',
+        verbose_name='変更後品種',
+        null=True,
+        blank=True,
+    )
+    reason = models.TextField(blank=True, default='', verbose_name='変更理由')
+    moved_entry_count = models.IntegerField(default=0, verbose_name='移動エントリ数')
+
+    class Meta:
+        verbose_name = '作付け計画品種変更履歴'
+        verbose_name_plural = '作付け計画品種変更履歴'
+        ordering = ['-changed_at', '-id']
+
+    def __str__(self):
+        old_name = self.old_variety.name if self.old_variety else '未設定'
+        new_name = self.new_variety.name if self.new_variety else '未設定'
+        return f'{self.field.name} {self.year}: {old_name} -> {new_name}'
+
+
 class RiceTransplantPlan(models.Model):
     name = models.CharField(max_length=200, verbose_name='計画名')
     year = models.IntegerField(verbose_name='年度')