fix: improve cluster detection to handle qdevice configurations (addresses #428)

- Add API validation for cluster nodes to filter out qdevice VMs
- Only include nodes with working Proxmox APIs in cluster endpoints
- Prevent connection failures when cluster has non-Proxmox participants
- Add detailed logging for cluster node validation process

This resolves issues where Proxmox clusters using corosync qdevice
(external quorum device) would fail to connect because Pulse tried
to connect to the qdevice VM which has no Proxmox API.

internal/api/config_handlers.go

@@ -151,6 +151,66 @@ type NodeResponse struct {
 	ClusterEndpoints   []config.ClusterEndpoint `json:"clusterEndpoints,omitempty"`
 }
 
+// validateNodeAPI tests if a cluster node has a working Proxmox API
+// This helps filter out qdevice VMs and other non-Proxmox participants
+func validateNodeAPI(clusterNode proxmox.ClusterStatus, baseConfig proxmox.ClientConfig) bool {
+	// Determine the host to test - prefer IP if available, otherwise use node name
+	testHost := clusterNode.IP
+	if testHost == "" {
+		testHost = clusterNode.Name
+	}
+	
+	// Skip empty hostnames (shouldn't happen but be safe)
+	if testHost == "" {
+		return false
+	}
+	
+	// Create a test configuration for this specific node
+	testConfig := baseConfig
+	testConfig.Host = testHost
+	if !strings.HasPrefix(testConfig.Host, "http") {
+		testConfig.Host = fmt.Sprintf("https://%s:8006", testConfig.Host)
+	}
+	
+	// Use a very short timeout for validation - we just need to know if the API exists
+	testConfig.Timeout = 2 * time.Second
+	
+	log.Debug().
+		Str("node", clusterNode.Name).
+		Str("test_host", testConfig.Host).
+		Msg("Validating Proxmox API for cluster node")
+	
+	// Try to create a client and make a simple API call
+	testClient, err := proxmox.NewClient(testConfig)
+	if err != nil {
+		log.Debug().
+			Str("node", clusterNode.Name).
+			Err(err).
+			Msg("Failed to create test client for cluster node")
+		return false
+	}
+	
+	// Test with a simple API call that all Proxmox nodes should support
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+	
+	// Try to get the node version - this is a very lightweight API call
+	_, err = testClient.GetNodes(ctx)
+	if err != nil {
+		log.Debug().
+			Str("node", clusterNode.Name).
+			Err(err).
+			Msg("Node failed Proxmox API validation - likely not a Proxmox node")
+		return false
+	}
+	
+	log.Debug().
+		Str("node", clusterNode.Name).
+		Msg("Node passed Proxmox API validation")
+	
+	return true
+}
+
 // detectPVECluster checks if a PVE node is part of a cluster and returns cluster information
 func detectPVECluster(clientConfig proxmox.ClientConfig, nodeName string) (isCluster bool, clusterName string, clusterEndpoints []config.ClusterEndpoint) {
 	tempClient, err := proxmox.NewClient(clientConfig)
@@ -194,6 +254,16 @@ func detectPVECluster(clientConfig proxmox.ClientConfig, nodeName string) (isClu
 			Msg("Detected Proxmox cluster")
 		
 		for _, clusterNode := range clusterNodes {
+			// Validate that this node actually has a working Proxmox API
+			// This filters out qdevice VMs and other non-Proxmox participants
+			if !validateNodeAPI(clusterNode, clientConfig) {
+				log.Debug().
+					Str("node", clusterNode.Name).
+					Str("ip", clusterNode.IP).
+					Msg("Skipping cluster node - no valid Proxmox API detected (likely qdevice or external node)")
+				continue
+			}
+			
 			endpoint := config.ClusterEndpoint{
 				NodeID:   clusterNode.ID,
 				NodeName: clusterNode.Name,
@@ -209,6 +279,13 @@ func detectPVECluster(clientConfig proxmox.ClientConfig, nodeName string) (isClu
 			clusterEndpoints = append(clusterEndpoints, endpoint)
 		}
 		
+		// Log the final count of valid Proxmox nodes found
+		log.Info().
+			Str("cluster", clusterName).
+			Int("total_discovered", len(clusterNodes)).
+			Int("valid_proxmox_nodes", len(clusterEndpoints)).
+			Msg("Cluster node validation complete")
+		
 		// Fallback if we couldn't get the cluster name
 		if clusterName == "" {
 			clusterName = "Unknown Cluster"