feat(back-end): email service and test

backend/src/main/java/com/printcalculator/BackendApplication.java

@@ -3,12 +3,14 @@ package com.printcalculator;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
 import org.springframework.scheduling.annotation.EnableScheduling;
+import org.springframework.scheduling.annotation.EnableAsync;
 import org.springframework.scheduling.annotation.EnableScheduling;
 import org.springframework.transaction.annotation.EnableTransactionManagement;
 
 @SpringBootApplication
 @EnableTransactionManagement
 @EnableScheduling
+@EnableAsync
 public class BackendApplication {
 
 	public static void main(String[] args) {