sdwan: add hub node selection and auto fallback to mesh

internal/server/sdwan_api.go

@@ -1,6 +1,7 @@
 package server
 
 import (
+	"errors"
 	"log"
 	"net/netip"
 
@@ -24,6 +25,15 @@ func (s *Server) SetSDWAN(cfg protocol.SDWANConfig) error {
 }
 
 func (s *Server) SetSDWANTenant(tenantID int64, cfg protocol.SDWANConfig) error {
+	if cfg.Mode == "hub" {
+		if cfg.HubNode == "" {
+			return errors.New("hub mode requires hubNode")
+		}
+		hub := s.GetNode(cfg.HubNode)
+		if hub == nil || !hub.IsOnline() || hub.TenantID != tenantID || !hub.RelayEnabled {
+			return errors.New("hub node must be online and relay-enabled")
+		}
+	}
 	if err := s.sdwan.saveTenant(tenantID, cfg); err != nil {
 		return err
 	}