fix(models): 修改TodayPlan speaker字段的删除行为和默认值

将speaker字段的on_delete行为从SET_NULL改为CASCADE，并设置默认值为1
删除旧的迁移文件0005_initial_speaker.py，使用新的0005_add_speaker_fields.py替代

core/migrations/0005_add_speaker_fields.py

@@ -1,4 +1,4 @@
-# Generated by Django 5.1.4
+# Generated by Django 5.1.4 on 2026-01-23 13:04
 
 import django.db.models.deletion
 from django.db import migrations, models

core/models.py

@@ -144,7 +144,7 @@ class TodayPlan(models.Model):
     """今日计划表"""
     date = models.DateField(default=timezone.now, verbose_name="日期")
     content = models.TextField(verbose_name="内容")
-    speaker = models.ForeignKey(FamilyMember, on_delete=models.SET_NULL, null=True, blank=True, verbose_name="发言人")
+    speaker = models.ForeignKey(FamilyMember, on_delete=models.CASCADE, default=1, verbose_name="发言人")
     priority = models.ForeignKey(Priority, on_delete=models.CASCADE, default=2, verbose_name="优先级")
     type = models.ForeignKey(PlanType, on_delete=models.CASCADE, default=4, verbose_name="类型")
     status = models.ForeignKey(Status, on_delete=models.CASCADE, default=1, verbose_name="状态")