fix: 修复 OSS 配额检查的语法错误

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/server.js

@@ -3049,7 +3049,7 @@ app.get('/api/files/upload-signature', authMiddleware, async (req, res) => {
       if (currentUsed + fileSize > ossQuota) {
         return res.status(400).json({
           success: false,
-          message: 
+          message: "OSS 存储空间不足，已用 " + (currentUsed / 1024 / 1024 / 1024).toFixed(2) + "GB / " + (ossQuota / 1024 / 1024 / 1024).toFixed(2) + "GB"
         });
       }
     } catch (usageErr) {
@@ -3201,7 +3201,7 @@ app.get('/api/files/download-url', authMiddleware, async (req, res) => {
       if (currentUsed + fileSize > ossQuota) {
         return res.status(400).json({
           success: false,
-          message: 
+          message: "OSS 存储空间不足，已用 " + (currentUsed / 1024 / 1024 / 1024).toFixed(2) + "GB / " + (ossQuota / 1024 / 1024 / 1024).toFixed(2) + "GB"
         });
       }
     } catch (usageErr) {