Fix RSA private key parsing to support both PKCS#1 and PKCS#8 formats

[jeffallen]

• Fix notification-backend RSA private key parsing error in loadPrivateKey()
  • Replace single x509.ParsePKCS1PrivateKey() call with dual-format support
  • Try PKCS#1 format first (legacy "-----BEGIN RSA PRIVATE KEY-----" headers)
  • Fall back to PKCS#8 format for modern OpenSSL-generated keys ("-----BEGIN PRIVATE KEY-----")
  • Add proper error handling with descriptive messages for both format attempts
  • Validate parsed key is actually RSA type when using PKCS#8 parsing
• Resolve "use ParsePKCS8PrivateKey instead for this key format" error
  • Modern OpenSSL genrsa command generates PKCS#8 format by default
  • Previous code only supported legacy PKCS#1 format causing parse failures
  • New implementation maintains backward compatibility with existing PKCS#1 keys
• Generate new 4096-bit RSA key pair for consistent cryptographic security
  • Update root public_key.pem with new public key for app-backend usage
  • Update demo-app/app/src/main/assets/public_key.pem for Android encryption
  • Keys generated using standard OpenSSL commands per README instructions
• Test successful notification-backend startup with both key formats
  • Firebase service account key (key.json) parsing already working correctly
  • RSA private key parsing now successful: "RSA private key loaded successfully"
  • Public key hash computation working: "Public key hash computed"
  • Server starts successfully on port 8080 with all endpoints active
• Maintain complete backward compatibility with existing PKCS#1 keys
• Fix resolves startup failure preventing notification-backend from functioning

Change-ID: s17cd5b682d740ba5k