feat(security): add security.md

SECURITY.md

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+# Security Policy
+
+## Contact Information
+
+If you discover a security vulnerability or have concerns about the security of Reticulum MeshChatX, please contact the lead developer using the following methods in order of preference:
+
+1. **LXMF**: `7cc8d66b4f6a0e0e49d34af7f6077b5a` (Ideally)
+2. **XMPP**: `ivan@chat.quad4.io`
+3. **Email**: `ivan@quad4.io`
+
+## Security Overview
+
+Reticulum MeshChatX is designed with a high degree of security in mind, leveraging multiple layers of protection and modern security practices. Detailed security enhancements are documented in the [CHANGELOG.md](CHANGELOG.md) and [README.md](README.md).
+
+### Core Security Features
+
+- **ASAR Integrity Validation**: Utilizes Electron 39 features to protect the application against tampering.
+- **Backend Binary Verification**: Generates a SHA-256 manifest of the unpacked Python backend during build and verifies it on every startup.
+- **Data-at-Rest Integrity Monitoring**: Snapshots the state of identities and database files on clean shutdown and warns if they were modified while the app was closed.
+- **3-Layer CSP Hardening**: Multi-layered Content Security Policy protection across the entire application stack:
+    1. **Backend Server CSP**: Applied via security middleware to all HTTP responses.
+    2. **Electron Session CSP**: Shell-level fallback CSP applied via `webRequest.onHeadersReceived`.
+    3. **Loading Screen CSP**: Bootloader CSP defined in HTML meta tags.
+- **Hardened Electron Environment**: Hardened security by disabling `runAsNode` and `nodeOptions` environment variables via Electron Fuses.
+- **Rootless Docker Images**: Support for running in restricted environments with rootless container images.
+
+### Automated Security Measures
+
+The project employs continuous security monitoring and testing:
+
+- **Security Scanning**: Automated daily scans using OSV-Scanner and Trivy for container image vulnerabilities.
+- **Pinned Actions**: All CI/CD workflows use pinned actions with full URLs to forked, vetted actions hosted on our Gitea instance (`git.quad4.io`) to prevent supply chain attacks.
+- **Extensive Testing & Fuzzing**: Comprehensive backend benchmarking suite with high-precision timing, memory delta tracking, and extreme stress modes to ensure stability and prevent resource exhaustion.
+- **Linting & Code Quality**: Strict linting rules and static analysis are enforced on every push.
+