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/0010_remove_conversation_fk.py

@@ -0,0 +1,17 @@
+# Generated by Django 6.0.1 on 2026-03-31 06:52
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('submission', '0009_add_view_count'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='submission',
+            name='conversation',
+        ),
+    ]