dsh-gateway-plugin: Adding a Password Gateway for DeepSeek Harness Web

dsh-gateway-plugin is a DSH plugin (MIT license) maintained by laoin114514, designed to provide a password-protected access gateway for DeepSeek Harness Web. Since the DSH web interface itself lacks authentication and Harness is restricted by official requirements to bind only to 127.0.0.1, this plugin establishes a "password reverse proxy" in front of it: the gateway port serves as the sole external entry point. Apart from the login/initial setup pages, no content is accessible without authentication (unauthenticated pages are redirected with 302, APIs return 401, and WebSockets are rejected). Authenticated requests are proxied to Harness after rewriting the Host and Origin headers. It supports initial password setup, password modification (which invalidates all sessions), constant-time password comparison, and login failure rate limiting (default 5...).