docs: add SECURITY.md with security policy and best practices

Co-authored-by: DevPossible <1957432+DevPossible@users.noreply.github.com>

Author: Copilot

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We release patches for security vulnerabilities for the following versions:
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 0.8.x   | :white_check_mark: |
+| < 0.8   | :x:                |
+
+## Reporting a Vulnerability
+
+We take the security of LCDPossible seriously. If you believe you have found a security vulnerability, please report it to us as described below.
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+### How to Report
+
+1. **GitHub Security Advisories**: Use the [GitHub Security Advisories](https://github.com/DevPossible/LCDPossible/security/advisories/new) feature (preferred)
+2. **Email**: Contact the maintainers via GitHub discussions or create a private security advisory
+
+### What to Include
+
+Please include as much of the following information as possible:
+
+- Type of issue (e.g., buffer overflow, SQL injection, cross-site scripting, etc.)
+- Full paths of source file(s) related to the manifestation of the issue
+- The location of the affected source code (tag/branch/commit or direct URL)
+- Any special configuration required to reproduce the issue
+- Step-by-step instructions to reproduce the issue
+- Proof-of-concept or exploit code (if possible)
+- Impact of the issue, including how an attacker might exploit it
+
+### Response Timeline
+
+- We will acknowledge receipt of your vulnerability report within 3 business days
+- We will send a more detailed response within 7 days indicating next steps
+- We will keep you informed of the progress towards a fix
+- We will notify you when the vulnerability is fixed
+
+## Security Best Practices for Users
+
+### Configuration Files
+
+- **Never commit sensitive data** to version control:
+  - Use environment variables or local config files (*.Local.json)
+  - These are excluded by .gitignore: `*.env`, `secrets.json`, `appsettings.*.Local.json`
+
+### Proxmox API Integration
+
+- **Use API tokens** instead of username/password authentication
+- **Create read-only tokens** with minimal required permissions
+- **Use SSL/TLS** for API connections (disable `IgnoreSslErrors` only for testing with self-signed certificates)
+- **Rotate tokens regularly** as part of good security hygiene
+
+### File Permissions
+
+- On Linux/macOS, ensure configuration files have appropriate permissions:
+  ```bash
+  chmod 600 /etc/lcdpossible/appsettings.json
+  ```
+
+### Running as a Service
+
+- **Windows**: The service runs under LocalSystem by default. Consider using a dedicated service account
+- **Linux**: Use systemd with a dedicated user account (not root)
+- **macOS**: Use launchd with a dedicated user account
+
+## Known Security Considerations
+
+### USB Device Access
+
+LCDPossible requires direct USB HID device access:
+- **Linux**: Requires udev rules for unprivileged access (see installation documentation)
+- **Windows**: Requires administrator privileges for first-time device access
+- **macOS**: May require Security & Privacy settings adjustment
+
+### LibVLC Integration
+
+The video panel feature uses LibVLC for media playback:
+- Keep LibVLC updated to receive security patches
+- Be cautious when playing untrusted media files or streams
+
+### Web Panel
+
+The web panel uses PuppeteerSharp (headless browser):
+- Be cautious when rendering untrusted web content
+- Network-isolated environments are recommended for displaying external websites
+
+## Secure Development
+
+### For Contributors
+
+- Never commit secrets or credentials to the repository
+- Use the provided .gitignore patterns
+- Mask sensitive data in logs and CLI output
+- Validate and sanitize all external inputs
+- Follow principle of least privilege for API integrations
+- Use parameterized queries if database access is added in the future
+
+### Code Review Checklist
+
+- [ ] No hardcoded credentials or API keys
+- [ ] Sensitive data properly masked in logs
+- [ ] Input validation and sanitization
+- [ ] Proper error handling (no information leakage)
+- [ ] Dependencies checked for known vulnerabilities
+- [ ] SSL/TLS used for network communication
+- [ ] Proper resource disposal (IDisposable pattern)
+
+## Security Updates
+
+Security updates are released as soon as possible after a vulnerability is confirmed. We recommend:
+
+- Subscribe to GitHub release notifications
+- Enable Dependabot alerts for your fork
+- Keep LCDPossible updated to the latest version
+
+## Acknowledgments
+
+We appreciate the security research community's efforts to help keep LCDPossible secure. Responsible disclosure helps protect all users.