[GREEN] Add AsyncConfig for email task executor with configurable thread pool

src/main/java/com/safetypin/authentication/config/AsyncConfig.java

+package com.safetypin.authentication.config;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.scheduling.annotation.EnableAsync;
+import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
+
+import java.util.concurrent.Executor;
+
+@Configuration
+@EnableAsync
+public class AsyncConfig {
+    
+    @Bean(name = "emailTaskExecutor")
+    public Executor taskExecutor() {
+        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
+        executor.setCorePoolSize(2);
+        executor.setMaxPoolSize(5);
+        executor.setQueueCapacity(100);
+        executor.setThreadNamePrefix("EmailThread-");
+        executor.initialize();
+        return executor;
+    }
+}