feat(security): add security.txt for reporting

issues

Introduced a security.txt file to provide a standard way for security
researchers to report vulnerabilities. The file specifies contact
information, preferred languages, and encryption details for secure
communication. This addition aligns with best practices for open-source
projects, enhancing our project's security posture.

- Establishes a clear communication channel for security issues.
- Encourages responsible disclosure by providing encryption options.
- Helps to streamline the vulnerability reporting process.

This change is in line with the security recommendations for modern web
services and applications.

security.txt

@@ -0,0 +1,15 @@
+-----BEGIN PGP SIGNED MESSAGE-----
+Hash: SHA256
+
+Contact: mailto:support@private.coffee
+Preferred-Languages: en, de
+Encryption: https://security.private.coffee/security@private.coffee.pub
+Canonical: https://security.private.coffee/security.txt
+Signed by: https://security.private.coffee/security@private.coffee.pub
+-----BEGIN PGP SIGNATURE-----
+
+iI4EARYIADYWIQR8+ffWeJVvJW3sPVIzxXDbF7H3lgUCZfQYvRgcc2VjdXJpdHlA
+cHJpdmF0ZS5jb2ZmZWUACgkQM8Vw2xex95ZAUwEA7tZJDZj7E/SLgxMguyKHZ0cg
+76oOQSEWzXumiVYlTOgBALQtAPHxGxbOxvDEJ1I8JzSUcYNV9ILx3ugWtM5FuoIN
+=e9Q0
+-----END PGP SIGNATURE-----