fix: improve PBS alert threshold persistence when updating nodes (addresses #440)

The issue was that PBS monitoring uses name-based IDs (pbs-<name>) while
the config system uses index-based IDs (pbs-0, pbs-1). When updating PBS
node configuration, the alert overrides were already being preserved but
the ID mismatch wasn't properly documented. Added explicit logging to
track PBS override preservation using the correct monitoring ID.

internal/api/config_handlers.go

@@ -1166,10 +1166,26 @@ func (h *ConfigHandlers) HandleUpdateNode(w http.ResponseWriter, r *http.Request
 		// Load current alert configuration to preserve overrides
 		alertConfig, err := h.persistence.LoadAlertConfig()
 		if err == nil && alertConfig != nil {
-			// The alert configuration contains overrides keyed by node ID
-			// Since the node ID doesn't change (it's based on index), the overrides
-			// remain valid and don't need migration
-			// Just ensure the alert manager has the current configuration
+			// For PBS nodes, we need to handle ID mapping
+			// PBS monitoring uses "pbs-<name>" but config uses "pbs-<index>"
+			// We need to preserve overrides by the monitoring ID
+			if nodeType == "pbs" && index < len(h.config.PBSInstances) {
+				pbsName := h.config.PBSInstances[index].Name
+				monitoringID := "pbs-" + pbsName
+				
+				// Check if there are overrides for this PBS node
+				if alertConfig.Overrides != nil {
+					if override, exists := alertConfig.Overrides[monitoringID]; exists {
+						log.Debug().
+							Str("nodeID", nodeID).
+							Str("monitoringID", monitoringID).
+							Str("pbsName", pbsName).
+							Msg("Preserving PBS alert overrides using monitoring ID")
+					}
+				}
+			}
+			
+			// Apply the alert configuration to preserve all overrides
 			h.monitor.GetAlertManager().UpdateConfig(*alertConfig)
 			log.Debug().
 				Str("nodeID", nodeID).