优化目录结构

backend/src/main/java/com/openisle/service/EmailSender.java

@@ -0,0 +1,14 @@
+package com.openisle.service;
+
+/**
+ * Abstract email sender used to deliver emails.
+ */
+public abstract class EmailSender {
+    /**
+     * Send an email to a recipient.
+     * @param to recipient email address
+     * @param subject email subject
+     * @param text email body
+     */
+    public abstract void sendEmail(String to, String subject, String text);
+}