fix: 添加 GenerationRecord.updated_at 字段（修复 Celery 僵尸任务恢复报错）

recover_stuck_tasks 和 poll_video_task 依赖 updated_at 字段判断僵尸任务，
但该字段未在 model 中定义，导致 Celery worker 持续报 FieldError，所有异步轮询失败。

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

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

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.29 on 2026-03-28 19:32
+
+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'),
+        ),
+    ]