fix: VisitSession 时间戳恢复 NOT NULL，测试补显式时间（修正计划的测试笔误）

Co-Authored-By: Claude Code <noreply@anthropic.com>

server/tracker/migrations/0001_initial.py

@@ -1,4 +1,4 @@
-# Generated by Django 5.2.17 on 2026-09-15 03:44
+# Generated by Django 5.2.17 on 2026-09-15 03:46
 
 import django.db.models.deletion
 import tracker.models
@@ -65,8 +65,8 @@ class Migration(migrations.Migration):
             fields=[
                 ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                 ('session_id', models.CharField(max_length=64)),
-                ('started_at', models.DateTimeField(blank=True, null=True)),
-                ('ended_at', models.DateTimeField(blank=True, null=True)),
+                ('started_at', models.DateTimeField()),
+                ('ended_at', models.DateTimeField()),
                 ('referrer_domain', models.CharField(blank=True, default='', max_length=200)),
                 ('utm', models.JSONField(blank=True, default=dict)),
                 ('landing_path', models.CharField(blank=True, default='', max_length=500)),