按 Mimosa 安全扫描建议处理 e2e 测试工具的存量高危项：
- e2e 集群的 root/admin 密码、会话密钥、同步 API Key 改为环境变量
  优先（E2E_ROOT_PASSWORD/E2E_ADMIN_PASSWORD/E2E_SESSION_SECRET/
  E2E_SYNC_API_KEY），本地回退值仅服务于 127.0.0.1 测试集群，
  Go 与 Playwright 两侧默认值保持一致
- killWindowsProcessTree 对 pid 强制正整数校验后再交给 taskkill，
  消除命令注入误报入口

- relay: SSE stream error exposure improvements (Claude/Gemini/OpenAI)
- test: add mock capture server for E2E relay capture testing

Co-Authored-By: Claude <noreply@anthropic.com>

Add candidate user selection and explicit_ids batch creation flow for overseas migration. Also add cancellable batches to release selected-user locks and extend backend, frontend, and e2e coverage for the new flow.

Co-Authored-By: Codex <noreply@anthropic.com>

Co-Authored-By: Codex <noreply@anthropic.com>