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/account/migrations/0003_user_classname.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.1 on 2026-03-18 02:33
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('account', '0002_alter_profile_total_score'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='user',
+            name='classname',
+            field=models.CharField(blank=True, default='', max_length=50, verbose_name='班级'),
+        ),
+    ]