feat: add user authentication system

- User model (users table) with bcrypt password hashing
- JWT-based login API (POST /api/auth/login, GET /api/auth/me)
- Default admin account (admin/admin123) auto-created on startup
- Login page with centered dark-themed login card
- Route guards + Axios interceptors for token management
- Login page renders without sidebar (standalone layout)
- Gunicorn --preload to avoid worker DDL race condition

backend/app/models/__init__.py

@@ -5,5 +5,7 @@ from app.models.task import ScheduledTask  # noqa: F401
 from app.models.execution import TaskExecution  # noqa: F401
 from app.models.notification import Alert, AlertRule, NotificationChannel  # noqa: F401
 from app.models.system_config import SystemConfig  # noqa: F401
+from app.models.user import User  # noqa: F401
 
-__all__ = ["Agent", "ScheduledTask", "TaskExecution", "NotificationChannel", "AlertRule", "Alert", "SystemConfig"]
+__all__ = ["Agent", "ScheduledTask", "TaskExecution", "NotificationChannel",
+           "AlertRule", "Alert", "SystemConfig", "User"]

backend/app/models/user.py

@@ -0,0 +1,25 @@
+"""SQLAlchemy models — User (dashboard login)."""
+
+from datetime import datetime
+
+from sqlalchemy import Boolean, DateTime, String, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.database import Base
+
+
+class User(Base):
+    """Dashboard user for login authentication."""
+    __tablename__ = "users"
+
+    id: Mapped[int] = mapped_column(primary_key=True, autoincrement=True)
+    username: Mapped[str] = mapped_column(String(64), unique=True, nullable=False, comment="登录用户名")
+    password_hash: Mapped[str] = mapped_column(String(256), nullable=False, comment="bcrypt 密码哈希")
+    display_name: Mapped[str] = mapped_column(String(128), default="", comment="显示名称")
+    email: Mapped[str] = mapped_column(String(128), default="", comment="邮箱")
+    is_active: Mapped[bool] = mapped_column(Boolean, default=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now())
+    updated_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now(), onupdate=func.now())
+
+    def __repr__(self) -> str:
+        return f"<User {self.username}>"