fix: 重新添加 updated_at migration（修复线上 IntegrityError）

Model 有 updated_at 字段但线上数据库缺少该列，INSERT 时报
"Field 'updated_at' doesn't have a default value"，导致视频生成 500。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/apps/generation/migrations/0014_add_updated_at_to_record.py

@@ -0,0 +1,16 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('generation', '0013_add_video_token_price'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='generationrecord',
+            name='updated_at',
+            field=models.DateTimeField(auto_now=True, verbose_name='更新时间'),
+        ),
+    ]