零 Node 部署、超级管理员，并完善本地构建发布文档。

- FastAPI 单进程托管 frontend/dist，systemd 替代 PM2

- 超级管理员 admin、注册开关与用户管理

- README/DEPLOY/USAGE 说明：改代码须本地构建 dist 后 push，服务器 update.sh

- 提交 frontend/dist 与 build-frontend 脚本

backend/app/routers/settings.py

@@ -0,0 +1,24 @@
+from fastapi import APIRouter, Depends
+from sqlalchemy.orm import Session
+
+from app.core.database import get_db
+from app.models.user import SystemSettings
+from app.schemas import PublicSettingsOut
+
+router = APIRouter(prefix="/settings", tags=["settings"])
+
+
+def get_or_create_settings(db: Session) -> SystemSettings:
+    row = db.get(SystemSettings, 1)
+    if row is None:
+        row = SystemSettings(id=1, registration_enabled=True)
+        db.add(row)
+        db.commit()
+        db.refresh(row)
+    return row
+
+
+@router.get("/public", response_model=PublicSettingsOut)
+def public_settings(db: Session = Depends(get_db)):
+    row = get_or_create_settings(db)
+    return PublicSettingsOut(registration_enabled=row.registration_enabled)