Introduce Chat API with message handling, server state updates, and IP-based security restrictions.

backend/src/main/resources/application.properties

@@ -18,3 +18,4 @@ spring.mail.password=${MAIL_PASSWORD}
 spring.mail.properties.mail.smtp.auth=true
 spring.mail.properties.mail.smtp.ssl.enable=true
 spring.mail.properties.mail.smtp.socketFactory.class=javax.net.ssl.SSLSocketFactory
+chat.allowed-ip=${ALLOWED_IP:10.0.0.0/24}