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/services/seed.py

@@ -1,6 +1,8 @@
 from sqlalchemy.orm import Session
 
-from app.models.user import Subject
+from app.core.config import settings
+from app.core.security import get_password_hash
+from app.models.user import Subject, SystemSettings, User
 
 DEFAULT_SUBJECTS = [
     "语文",
@@ -21,3 +23,22 @@ def seed_subjects(db: Session) -> None:
         if name not in existing:
             db.add(Subject(name=name))
     db.commit()
+
+
+def seed_admin_and_settings(db: Session) -> None:
+    if db.get(SystemSettings, 1) is None:
+        db.add(SystemSettings(id=1, registration_enabled=True))
+
+    admin = db.query(User).filter(User.username == settings.ADMIN_DEFAULT_USERNAME).first()
+    if admin is None:
+        db.add(
+            User(
+                username=settings.ADMIN_DEFAULT_USERNAME,
+                password_hash=get_password_hash(settings.ADMIN_DEFAULT_PASSWORD),
+                is_superuser=True,
+            )
+        )
+    elif not admin.is_superuser:
+        admin.is_superuser = True
+
+    db.commit()