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/0003_submission_conversation.py

@@ -0,0 +1,20 @@
+# Generated by Django 6.0.1 on 2026-03-04 11:03
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('prompt', '0001_initial'),
+        ('submission', '0002_remove_submission_referee_alter_submission_score_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='submission',
+            name='conversation',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='submissions', to='prompt.conversation', verbose_name='对话'),
+        ),
+    ]