Introduce pluggable email and image upload

src/main/java/com/openisle/service/CosImageUploader.java

@@ -0,0 +1,24 @@
+package com.openisle.service;
+
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.stereotype.Service;
+
+/**
+ * ImageUploader implementation using Tencent Cloud COS.
+ * For simplicity this demo just returns a URL composed of the base URL and file name.
+ */
+@Service
+public class CosImageUploader extends ImageUploader {
+
+    private final String baseUrl;
+
+    public CosImageUploader(@Value("${cos.base-url:https://example.com}") String baseUrl) {
+        this.baseUrl = baseUrl;
+    }
+
+    @Override
+    public String upload(byte[] data, String filename) {
+        // In a real implementation you would call COS SDK here
+        return baseUrl + "/" + filename;
+    }
+}