Fix nginx sub-services flickering with persistent caching

- Remove nginx_ prefix from site names in hierarchical structure
- Fix get_nginx_site_metrics to call correct internal method
- Implement same caching functionality as old working version
- Sites now stay visible continuously with 30s latency updates
- Preserve cached results between refresh cycles

agent/src/collectors/systemd.rs

@@ -101,7 +101,7 @@ impl SystemdCollector {
                     
                     let mut sub_services = Vec::new();
 
-                    // Collect sub-services for specific services
+                    // Sub-service metrics for specific services (always include cached results)
                     if service_name.contains("nginx") && active_status == "active" {
                         let nginx_sites = self.get_nginx_site_metrics();
                         for (site_name, latency_ms) in nginx_sites {
@@ -216,7 +216,7 @@ impl SystemdCollector {
         Ok(state.monitored_services.clone())
     }
 
-    /// Get nginx site metrics, checking them if cache is expired
+    /// Get nginx site metrics, checking them if cache is expired (like old working version)
     fn get_nginx_site_metrics(&self) -> Vec<(String, f32)> {
         let mut state = self.state.write().unwrap();
 
@@ -574,11 +574,11 @@ impl SystemdCollector {
         for (site_name, url) in &discovered_sites {
             match self.check_site_latency(url) {
                 Ok(latency_ms) => {
-                    sites.push((format!("nginx_{}", site_name), latency_ms));
+                    sites.push((site_name.clone(), latency_ms));
                 }
                 Err(_) => {
                     // Site is unreachable - use -1.0 to indicate error
-                    sites.push((format!("nginx_{}", site_name), -1.0));
+                    sites.push((site_name.clone(), -1.0));
                 }
             }
         }