OSS 结果文件下载分流：新增 aiimage.oss.download-endpoint 配置，result/ 前缀对象生成独立下载域名直链（默认未配置时回退原行为）

backend-java/src/main/java/com/nanri/aiimage/config/OssProperties.java

@@ -9,6 +9,7 @@ public class OssProperties {
     private String region;
     private String endpoint;
     private String publicEndpoint;
+    private String downloadEndpoint;
     private String bucket;
     private String imageVideoBucket;
     private String digitalHumanBucket;