docs: add security policy with private vulnerability reporting

Closes #7812

.github/SECURITY.md

@@ -0,0 +1,103 @@
+# Security Policy
+
+[English](#english) | [中文](#中文)
+
+## English
+
+### Reporting a Vulnerability
+
+**Please do not report security vulnerabilities through public GitHub issues, pull requests, or discussions.**
+
+Report them privately through GitHub's private vulnerability reporting:
+
+**[Report a vulnerability](https://github.com/Wei-Shaw/sub2api/security/advisories/new)**
+
+(Repository → **Security** tab → **Report a vulnerability**.) Only the maintainers can see the report, and the advisory becomes the place to discuss, fix, and eventually publish the issue.
+
+A useful report includes:
+
+- The affected component and version (release tag or commit)
+- Impact: what an attacker can do and which privileges they need
+- Minimal reproduction steps or a proof of concept
+- Any non-default configuration required to trigger the issue
+- A suggested fix, if you have one
+
+### Supported Versions
+
+Sub2API releases frequently, and fixes ship in a new release rather than being backported.
+
+| Version | Supported |
+| ------- | --------- |
+| Latest release | ✅ |
+| Older releases | ❌ |
+
+Please confirm that the issue reproduces on the latest release or on `main` before reporting.
+
+### Scope
+
+In scope: the Sub2API backend, frontend, official Docker images, and deployment files in this repository.
+
+Usually out of scope:
+
+- Issues that require already-compromised admin credentials or server access
+- Risks arising from explicitly weakening documented security settings (for example, disabling `security.url_allowlist`)
+- Vulnerabilities in upstream AI providers or third-party dependencies with no Sub2API-specific impact (please report those upstream)
+- Missing best-practice headers or scanner output without a demonstrated impact
+
+### Disclosure Process
+
+1. We acknowledge the report and assess its validity and severity.
+2. We develop a fix privately in the advisory and may ask you to verify it.
+3. We publish a release containing the fix, then publish the GitHub Security Advisory (with a CVE when applicable), crediting you unless you prefer to stay anonymous.
+
+Please keep details private until the advisory is published. This is a volunteer-maintained project, so response times are best effort.
+
+## 中文
+
+### 报告漏洞
+
+**请不要通过公开的 Issue、Pull Request 或 Discussion 报告安全漏洞。**
+
+请使用 GitHub 私密漏洞报告功能提交：
+
+**[提交漏洞报告](https://github.com/Wei-Shaw/sub2api/security/advisories/new)**
+
+（仓库 → **Security** 标签页 → **Report a vulnerability**。）报告仅维护者可见，后续的讨论、修复与公告发布都在该安全公告中进行。
+
+报告中建议包含：
+
+- 受影响的组件与版本（Release tag 或 commit）
+- 影响：攻击者能做什么、需要什么权限
+- 最小复现步骤或 PoC
+- 触发问题所需的非默认配置
+- 修复建议（如有）
+
+### 支持的版本
+
+Sub2API 发版频繁，安全修复随新版本发布，不向旧版本回移。
+
+| 版本 | 是否支持 |
+| ---- | -------- |
+| 最新 Release | ✅ |
+| 更早的版本 | ❌ |
+
+报告前请先确认问题在最新 Release 或 `main` 上仍可复现。
+
+### 范围
+
+范围内：本仓库中的 Sub2API 后端、前端、官方 Docker 镜像与部署文件。
+
+通常不在范围内：
+
+- 需要事先已掌握管理员凭据或服务器权限才能利用的问题
+- 因主动关闭文档中的安全配置（如关闭 `security.url_allowlist`）而产生的风险
+- 上游 AI 服务商或第三方依赖自身的漏洞，且对 Sub2API 无特定影响（请向上游报告）
+- 仅缺少最佳实践响应头或扫描器输出，但未证明实际影响
+
+### 披露流程
+
+1. 我们确认收到报告，并评估其有效性与严重程度。
+2. 我们在安全公告中私下开发修复，可能请你协助验证。
+3. 我们先发布包含修复的版本，再公开 GitHub 安全公告（适用时申请 CVE），并致谢报告者（如你希望匿名则不署名）。
+
+在公告发布前请勿公开漏洞细节。本项目由志愿者维护，响应时间尽力而为。