优化目录结构

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

@@ -0,0 +1,14 @@
+package com.openisle.service;
+
+/**
+ * Abstract service for verifying CAPTCHA tokens.
+ */
+public abstract class CaptchaService {
+    /**
+     * Verify the CAPTCHA token sent from client.
+     *
+     * @param token CAPTCHA token
+     * @return true if token is valid
+     */
+    public abstract boolean verify(String token);
+}