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/0007_remove_nominated.py

@@ -0,0 +1,15 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("submission", "0006_add_raw_score_nominated"),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name="submission",
+            name="nominated",
+        ),
+    ]