feat: add download_token field to Config model

backend/app/models.py

@@ -1,3 +1,4 @@
+import secrets
 from datetime import datetime
 from sqlalchemy import (
     Boolean, DateTime, ForeignKey, Integer, String, Text, UniqueConstraint, func
@@ -27,6 +28,11 @@ class Config(Base):
     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())
+    download_token: Mapped[str] = mapped_column(
+        String(64),
+        nullable=False,
+        default=lambda: secrets.token_urlsafe(32),
+    )
     owner_id: Mapped[int] = mapped_column(Integer, ForeignKey("users.id"), nullable=False)
 
     owner: Mapped["User"] = relationship("User", back_populates="configs")