security: add SECURITY.md policy and enable GitHub security features

- SECURITY.md: reporting instructions (private advisory + email),
  scope, disclosure policy, post-install hardening tips
- Enabled via API: Dependabot alerts, private vulnerability reporting

SECURITY.md

@@ -0,0 +1,76 @@
+# Security Policy
+
+## Supported Versions
+
+Only the latest release of NixPanel receives security fixes.
+
+| Version | Supported |
+|---------|-----------|
+| Latest stable | ✅ |
+| Older releases | ❌ |
+
+We strongly recommend always running the latest version. The installer is self-updating and will always pull the current release.
+
+## Reporting a Vulnerability
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+### Option 1 — GitHub Private Vulnerability Reporting (preferred)
+
+Use GitHub's built-in private reporting:  
+[Report a vulnerability](https://github.com/NixReaper/nixpanel/security/advisories/new)
+
+This keeps the report confidential until a fix is released.
+
+### Option 2 — Email
+
+Send details to **[email protected]**
+
+Include:
+- A description of the vulnerability
+- Steps to reproduce
+- Affected versions
+- Any proof-of-concept code (if available)
+
+We will acknowledge your report within **48 hours** and aim to release a fix within **14 days** for critical issues.
+
+## Scope
+
+Issues we consider in-scope:
+
+- Remote code execution
+- Privilege escalation
+- Authentication bypass
+- SQL injection
+- Sensitive data exposure (credentials, license keys, private keys)
+- Cross-site scripting (XSS) in the control panel UI
+- Server-side request forgery (SSRF)
+
+Out of scope:
+
+- Vulnerabilities in software NixPanel installs but does not control (Apache, MariaDB, PHP, etc.) — report those to the respective upstream projects
+- Issues requiring physical access to the server
+- Social engineering attacks
+
+## Security Best Practices
+
+After installing NixPanel:
+
+1. **Run certbot immediately** — never leave the panel on plain HTTP in production
+   ```bash
+   certbot --apache
+   ```
+2. **Restrict panel ports** — firewall ports 2083 and 2087 to trusted IPs only
+3. **Keep the system updated** — `apt upgrade` regularly
+4. **Use strong admin passwords** — the panel enforces Argon2id hashing
+5. **Monitor logs** — `journalctl -u nixpanel -f` and `/var/log/nixpanel/`
+
+## Disclosure Policy
+
+We follow **coordinated disclosure**. We ask that you:
+
+- Give us reasonable time to fix the issue before public disclosure
+- Not access or modify other users' data during testing
+- Not perform denial-of-service testing
+
+We will credit researchers in release notes unless you prefer to remain anonymous.