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/0008_add_zone.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.1 on 2026-03-30 11:47
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('submission', '0007_remove_nominated'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='submission',
+            name='zone',
+            field=models.CharField(blank=True, choices=[('featured', '精选'), ('low', '待改进'), ('pending', '待评')], db_index=True, default=None, max_length=10, null=True, verbose_name='分区'),
+        ),
+    ]