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/prompt/migrations/0006_message_submission_cascade_delete.py

@@ -0,0 +1,20 @@
+# Generated by Django 6.0.1 on 2026-04-16 06:15
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('prompt', '0005_message_add_submission_fk'),
+        ('submission', '0010_remove_conversation_fk'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='message',
+            name='submission',
+            field=models.OneToOneField(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='source_message', to='submission.submission'),
+        ),
+    ]