chore: 将后端移动到 api/ 目录，准备合并为单仓库

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/submission/migrations/0012_submissionaward_is_stale.py

@@ -0,0 +1,16 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("submission", "0011_add_award_submissionaward"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="submissionaward",
+            name="is_stale",
+            field=models.BooleanField(default=False, verbose_name="有更新提交"),
+        ),
+    ]