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

@@ -0,0 +1,38 @@
+# Generated by Django 5.1.4 on 2026-01-23 13:04
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0004_systemconfig_sender_email'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='FamilyMember',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=20, unique=True, verbose_name='姓名')),
+                ('created_at', models.DateTimeField(auto_now_add=True, verbose_name='创建时间')),
+                ('updated_at', models.DateTimeField(auto_now=True, verbose_name='更新时间')),
+            ],
+            options={
+                'verbose_name': '家庭成员',
+                'verbose_name_plural': '家庭成员',
+                'ordering': ['name'],
+            },
+        ),
+        migrations.AddField(
+            model_name='insightrecord',
+            name='speaker',
+            field=models.ForeignKey(default=1, on_delete=django.db.models.deletion.CASCADE, to='core.familymember', verbose_name='发言人'),
+        ),
+        migrations.AddField(
+            model_name='todayplan',
+            name='speaker',
+            field=models.ForeignKey(default=1, on_delete=django.db.models.deletion.CASCADE, to='core.familymember', verbose_name='发言人'),
+        ),
+    ]