Fix NODE column in backups to show actual guest node

Related to discussion #577

When backups are stored on shared storage accessible from multiple nodes,
the backup polling code was incorrectly assigning the backup to whichever
node it was discovered on during the scan, rather than the node where the
VM/container actually resides.

This fix:
- Builds a lookup map of VMID -> actual node at the start of backup polling
- Uses this map to assign the correct node for guest backups (VMID > 0)
- Preserves existing behavior for host backups (VMID == 0)
- Falls back to the queried node if the guest is not found in the map

This ensures the NODE column accurately reflects which node hosts each
guest, matching the information displayed on the main page.

internal/monitoring/monitor.go

@@ -7735,6 +7735,20 @@ func (m *Monitor) pollStorageBackupsWithNodes(ctx context.Context, instanceName
 	contentFailures := 0                // Number of failed storage content fetches
 	storageQueryErrors := 0             // Number of nodes where storage list could not be queried
 
+	// Build guest lookup map to find actual node for each VMID
+	snapshot := m.state.GetSnapshot()
+	guestNodeMap := make(map[int]string) // VMID -> actual node name
+	for _, vm := range snapshot.VMs {
+		if vm.Instance == instanceName {
+			guestNodeMap[vm.VMID] = vm.Node
+		}
+	}
+	for _, ct := range snapshot.Containers {
+		if ct.Instance == instanceName {
+			guestNodeMap[int(ct.VMID)] = ct.Node
+		}
+	}
+
 	// For each node, get storage and check content
 	for _, node := range nodes {
 		if nodeEffectiveStatus[node.Node] != "online" {
@@ -7828,8 +7842,15 @@ func (m *Monitor) pollStorageBackupsWithNodes(ctx context.Context, instanceName
 					backupType = "qemu"
 				}
 
-				// Always use the actual node name
+				// Determine the correct node: for guest backups (VMID > 0), use the actual guest's node
+				// For host backups (VMID == 0), use the node where the backup was found
 				backupNode := node.Node
+				if content.VMID > 0 {
+					if actualNode, found := guestNodeMap[content.VMID]; found {
+						backupNode = actualNode
+					}
+					// If not found in map, fall back to queried node (shouldn't happen normally)
+				}
 				isPBSStorage := strings.HasPrefix(storage.Storage, "pbs-") || storage.Type == "pbs"
 
 				// Check verification status for PBS backups