Remove hardcoded discovery interval in systemd collector

- Use config.interval_seconds instead of hardcoded 300 seconds
- Discovery now happens every 10 seconds (configurable) instead of 5 minutes
- Follows configuration-driven architecture requirements

agent/src/collectors/systemd.rs

@@ -52,7 +52,7 @@ impl SystemdCollector {
                 monitored_services: Vec::new(),
                 service_status_cache: std::collections::HashMap::new(),
                 last_discovery_time: None,
-                discovery_interval_seconds: 300, // 5 minutes
+                discovery_interval_seconds: config.interval_seconds,
                 nginx_site_metrics: Vec::new(),
                 last_nginx_check_time: None,
                 nginx_check_interval_seconds: 30, // 30 seconds for nginx sites