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/apps.py

@@ -0,0 +1,7 @@
+from django.apps import AppConfig
+
+
+class AccountConfig(AppConfig):
+    default_auto_field = "django.db.models.BigAutoField"
+    name = "account"
+    verbose_name = "用户"