chore: ThreadPoolTaskExecutor의 corePoolSize를 2로 설정

src/main/java/balancetalk/global/config/AsyncConfig.java

@@ -15,6 +15,7 @@ public class AsyncConfig implements AsyncConfigurer {
     @Override
     public Executor getAsyncExecutor() {
         ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
+        executor.setCorePoolSize(2);
         executor.setThreadNamePrefix("Async task - ");
         executor.initialize();
         return executor;