fix: address code review findings

cmd/naviwatcher/main.go

@@ -84,6 +84,9 @@ func NewApp(ctx context.Context, cfg *config.Config) (*App, error) {
 
 // Close cleans up all application resources in reverse order of initialization.
 func (a *App) Close() {
+	if a.mbClient != nil {
+		a.mbClient.Close()
+	}
 	if a.db != nil {
 		if err := a.db.Close(); err != nil {
 			log.Printf("Error closing database: %v", err)