fix: 七牛云上传缺少 File.Name 导致上传失败

UploadReader 调用时未设置 ObjectOptions.FileName，SDK 内部
PostObject 校验 File.Name 不能为空，返回 missing required field 错误。
从 key 中提取文件名填入 FileName 字段即可。

backend/internal/service/storage.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"fmt"
 	"net/url"
+	"path/filepath"
 	"strings"
 	"time"
 
@@ -57,6 +58,7 @@ func (s *StorageService) Upload(ctx context.Context, key string, data []byte) (s
 	err = uploadMgr.UploadReader(ctx, bytes.NewReader(data), &uploader.ObjectOptions{
 		BucketName: s.bucket,
 		ObjectName: &key,
+		FileName:   filepath.Base(key),
 	}, nil)
 	if err != nil {
 		l.Error("qiniu upload failed", "key", key, "error", err)