fix: address network probe code quality issues

- Use shared http.Client in ProbeManager to avoid connection/transport leak
- Skip probe goroutine and agent request when system has no enabled probes
- Validate HTTP probe target URL scheme (http:// or https://) on creation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/hub/systems/system.go

@@ -169,7 +169,9 @@ func (sys *System) update() error {
 	}
 
 	// Fetch and save network probe results
-	go sys.fetchAndSaveProbeResults()
+	if sys.hasEnabledProbes() {
+		go sys.fetchAndSaveProbeResults()
+	}
 
 	return err
 }