fix: precheck local share download quota at download-url stage

backend/server.js

@@ -5892,6 +5892,44 @@ app.post('/api/share/:code/download-url', shareRateLimitMiddleware, async (req,
 
     // 本地存储：继续走后端下载
     if (storageType !== 'oss') {
+      if (!ownerTrafficState.isUnlimited) {
+        const { StorageInterface } = require('./storage');
+        const userForStorage = buildStorageUserContext(shareOwner, {
+          current_storage_type: storageType
+        });
+        const storageInterface = new StorageInterface(userForStorage);
+        let storage = null;
+        try {
+          storage = await storageInterface.connect();
+          const fileStats = await storage.stat(normalizedFilePath);
+          const fileSize = Number(fileStats?.size || 0);
+          if (!Number.isFinite(fileSize) || fileSize <= 0) {
+            return res.status(503).json({
+              success: false,
+              message: getBusyDownloadMessage()
+            });
+          }
+          if (fileSize > ownerTrafficState.remaining) {
+            return res.status(503).json({
+              success: false,
+              message: getBusyDownloadMessage()
+            });
+          }
+        } catch (statError) {
+          if (statError && (statError.code === 'ENOENT' || String(statError.message || '').includes('不存在'))) {
+            return res.status(404).json({
+              success: false,
+              message: '文件不存在'
+            });
+          }
+          throw statError;
+        } finally {
+          if (storage) {
+            await storage.end();
+          }
+        }
+      }
+
       let downloadUrl = `${getSecureBaseUrl(req)}/api/share/${code}/download-file?path=${encodeURIComponent(normalizedFilePath)}`;
 
       if (share.share_password) {