Add endpoints, services, and security controls for particle file management, including save and download APIs.

backend/src/main/resources/application.properties

@@ -6,4 +6,6 @@ database.user=${DB_USER:root}
 database.password=${DB_PASSWORD:root}
 cors.allowed-origins=${CORS:https://alttd.com}
 login.secret=${LOGIN_SECRET:SET_TOKEN}
+particles.file_path=${user.home}/.altitudeweb/particles
+notification.server.url=10.0.0.107:8080
 logging.level.com.alttd.altitudeweb=INFO