fix: handle forward service rebind on self-occupied port

When UpdateService encounters bind address conflicts (port already in use),
the handler now automatically deletes existing forward services and retries
the AddService operation. This resolves issues where a forward's own stale
listener prevents the update.

- Add isBindAddressInUseError() to detect port bind conflicts
- Add rebindForwardServiceOnSelfOccupiedPort() for automatic cleanup and retry
- Add HasOtherForwardOnNodePort() repository method to verify port ownership
- Add unit tests for bind conflict detection

go-backend/internal/http/handler/control_plane_test.go

@@ -1,6 +1,7 @@
 package handler
 
 import (
+	"errors"
 	"reflect"
 	"testing"
 )
@@ -66,6 +67,21 @@ func TestIsAlreadyExistsMessage(t *testing.T) {
 	}
 }
 
+func TestIsBindAddressInUseError(t *testing.T) {
+	if !isBindAddressInUseError(errors.New("listen tcp [::]:10001: bind: address already in use")) {
+		t.Fatalf("address already in use should be detected")
+	}
+	if !isBindAddressInUseError(errors.New("listen tcp4 13.228.170.187:16765: bind: cannot assign requested address")) {
+		t.Fatalf("cannot assign requested address should be detected")
+	}
+	if isBindAddressInUseError(errors.New("service demo already exists")) {
+		t.Fatalf("already exists should not be treated as bind conflict")
+	}
+	if isBindAddressInUseError(nil) {
+		t.Fatalf("nil error should not be treated as bind conflict")
+	}
+}
+
 func TestBuildForwardServiceConfigs_UsesBindIPForListen(t *testing.T) {
 	forward := &forwardRecord{RemoteAddr: "1.2.3.4:80", Strategy: "fifo", TunnelID: 7}
 	node := &nodeRecord{TCPListenAddr: "[::]", UDPListenAddr: "[::]"}