fix(auth): persist auto-generated JWT secret to survive restarts (#2933)

* fix(auth): persist auto-generated JWT secret to survive restarts

  When AUTH_JWT_SECRET is not set, the auto-generated secret is now
  written to .deer-flow/.jwt_secret (mode 0600) and reused on subsequent
  starts. This prevents session invalidation on every restart while still
  allowing explicit AUTH_JWT_SECRET in .env to take precedence.

* Apply suggestions from code review

Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* fix the lint errors of backend

---------

Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

backend/docs/AUTH_UPGRADE.md

@@ -99,7 +99,7 @@ rm -f backend/.deer-flow/data/deerflow.db
 | `.deer-flow/users/{user_id}/memory.json` | 用户级 memory |
 | `.deer-flow/users/{user_id}/agents/{agent_name}/` | 用户自定义 agent 配置、SOUL 和 agent memory |
 | `.deer-flow/admin_initial_credentials.txt` | `reset_admin` 生成的新凭据文件（0600，读完应删除） |
-| `.env` 中的 `AUTH_JWT_SECRET` | JWT 签名密钥（未设置时自动生成临时密钥，重启后 session 失效） |
+| `.env` 中的 `AUTH_JWT_SECRET` | JWT 签名密钥（未设置时自动生成并持久化到 `.deer-flow/.jwt_secret`，重启后 session 保持） |
 
 ### 生产环境建议
 
@@ -137,4 +137,4 @@ python -c "import secrets; print(secrets.token_urlsafe(32))"
 | 启动后没看到密码 | 当前实现不在启动日志输出密码 | 首次安装访问 `/setup`；忘记密码用 `reset_admin` |
 | `/login` 自动跳到 `/setup` | 系统还没有 admin | 在 `/setup` 创建第一个 admin |
 | 登录后 POST 返回 403 | CSRF token 缺失 | 确认前端已更新 |
-| 重启后需要重新登录 | `AUTH_JWT_SECRET` 未持久化 | 在 `.env` 中设置固定密钥 |
+| 重启后需要重新登录 | `.jwt_secret` 文件被删除且 `.env` 未设置 `AUTH_JWT_SECRET` | 在 `.env` 中设置固定密钥 |