📝 docs: add SECURITY.md with vulnerability reporting policy (#13528)

📝 docs: add SECURITY.md with vulnerability reporting policy

Define supported versions, reporting guidelines, response timeline, scope (in/out), and disclosure policy for security vulnerabilities.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We only provide security fixes for the **latest 2.x release**. Older versions (including all 1.x releases) are end-of-life and will not receive patches.
+
+| Version      | Supported |
+| ------------ | --------- |
+| 2.x (latest) | ✅        |
+| 1.x          | ❌        |
+| 0.x          | ❌        |
+
+If you are running a 1.x deployment, we strongly recommend upgrading to the latest 2.x release.
+
+## Reporting a Vulnerability
+
+Please report security vulnerabilities through the GitHub Security Advisory ["Report a Vulnerability"](https://github.com/lobehub/lobehub/security/advisories/new) tab.
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+### Response Timeline
+
+- **Acknowledgement**: We aim to respond to all reports within **7 days**.
+- **Fix**: Confirmed vulnerabilities will be addressed within **30 days**.
+- **Urgent issues**: If you believe the vulnerability is critical and actively exploitable, you can reach out directly on Discord (`arvinxu`) for faster coordination.
+
+### What to Include
+
+A good vulnerability report should include:
+
+- A clear description of the issue and its potential impact
+- The affected version (must be the latest 2.x release)
+- Step-by-step reproduction instructions or a working PoC
+- Any relevant logs, screenshots, or code references
+
+## Scope
+
+### In Scope
+
+- Security issues affecting the **latest 2.x release** of LobeHub
+- Vulnerabilities in the **server-side deployment** (LobeHub Cloud or self-hosted server mode)
+- Issues that can be exploited **without requiring admin/owner access** to the deployment
+
+### Out of Scope (Not a Vulnerability)
+
+The following are considered **by design** or **out of scope** and will not be accepted as vulnerability reports:
+
+#### 1. End-of-Life Versions
+
+Any issue that only affects 1.x or earlier versions. This includes but is not limited to the `X-lobe-chat-auth` header mechanism, `webapi` route authentication, and other 1.x-specific architectures that have been completely removed in 2.x.
+
+#### 2. File Proxy Public Access (`/f/:id`)
+
+The file proxy endpoint `/f/:id` uses randomly generated, non-enumerable IDs as [capability URLs](https://www.w3.org/TR/capability-urls/). This is a deliberate design choice, similar to how S3 presigned URLs or Google Docs sharing links work. Knowing the URL grants access — this is by design, not an authorization bypass.
+
+#### 3. User Enumeration on Login Flows
+
+Endpoints such as `check-user` that indicate whether an account exists are part of the standard login UX. This is a common and intentional pattern used by most modern authentication flows.
+
+#### 4. Self-Hosted Client-Side API Key Storage
+
+In self-hosted client-side mode, users configure their own API keys which are stored in the browser's local storage. This is the expected behavior for client-side deployments where the user is both the operator and the consumer.
+
+#### 5. Issues Requiring Admin or Owner Privileges
+
+Actions that require administrative access to the deployment (e.g., environment variable configuration, server-side settings) are not considered security vulnerabilities, as the admin is already a trusted party.
+
+#### 6. Theoretical Attacks Without Practical Impact
+
+Reports based on theoretical attack scenarios without a working proof of concept against a realistic deployment, or issues that require unlikely preconditions (e.g., physical access to the server, pre-existing compromise of the host system).
+
+## Disclosure Policy
+
+- We follow [coordinated vulnerability disclosure](https://en.wikipedia.org/wiki/Coordinated_vulnerability_disclosure).
+- We will credit reporters in the security advisory unless they prefer to remain anonymous.
+- Please allow us reasonable time to address the issue before any public disclosure.
+
+## Contact
+
+- **Primary**: [GitHub Security Advisories](https://github.com/lobehub/lobehub/security/advisories/new)
+- **Urgent**: Discord — `arvinxu`