Add RFC 9116 compliant security.txt endpoint

- Implement /.well-known/security.txt endpoint for security vulnerability reporting
- Add SECURITY_CONTACT environment variable support
- Include proper HTTP headers (Content-Type, Cache-Control)
- Set automatic expiration date and preferred languages
- Add comprehensive security policy information
- Update .env.example with SECURITY_CONTACT variable
- Document security.txt endpoint in README.md with usage examples
- Follow RFC 9116 standard for responsible disclosure

.env.example

@@ -55,3 +55,4 @@ AWS_SECRET_ACCESS_KEY=your_aws_secret_key_here
 
 # Other API Keys (optional)
 BW_CLIENTSECRET=your_bw_client_secret_here
+SECURITY_CONTACT=security@stargirlnails.de  # E-Mail für Sicherheitsmeldungen