Fix: preserve /files prefix when proxying to dufs --path-prefix

nginx.conf

@@ -40,7 +40,7 @@ http {
         # dufs 文件服务 (公开，无需登录)
         # -----------------------------
         location /files/ {
-            proxy_pass http://127.0.0.1:5000/;
+            proxy_pass http://127.0.0.1:5000;
             proxy_http_version 1.1;
 
             proxy_set_header Host $host;