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/models/user.py

@@ -33,6 +33,7 @@ class User(Base):
     id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
     username: Mapped[str] = mapped_column(String(64), unique=True, index=True)
     password_hash: Mapped[str] = mapped_column(String(255))
+    is_superuser: Mapped[bool] = mapped_column(default=False)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
     )
@@ -128,3 +129,13 @@ class WrongQuestion(Base):
 
     student: Mapped["Student"] = relationship(back_populates="wrong_questions")
     subject: Mapped["Subject"] = relationship(back_populates="wrong_questions")
+
+
+class SystemSettings(Base):
+    __tablename__ = "system_settings"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, default=1)
+    registration_enabled: Mapped[bool] = mapped_column(default=True)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )