perf: async OSS upload in resumable complete endpoint

For OSS storage, the /api/upload/resumable/complete endpoint now returns
immediately after all chunks are assembled, then uploads to OSS in the
background. This eliminates the blocking re-upload delay for large files.

Local storage remains synchronous (already optimized with rename-first).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/server.js

@@ -6460,11 +6460,58 @@ app.post('/api/upload/resumable/complete', authMiddleware, async (req, res) => {
     });
     const storageInterface = new StorageInterface(storageUserContext);
     storage = await storageInterface.connect();
-    await storage.put(session.temp_file_path, session.target_path);
+
     if (sessionStorageType === 'oss') {
-      clearOssUsageCache(req.user.id);
+      // OSS 模式：先标记会话为 finalizing 并立即返回响应，后台异步上传到 OSS
+      UploadSessionDB.setStatus(session.session_id, 'finalizing', {
+        completed: false,
+        expiresAt: buildResumableUploadExpiresAt(30 * 60 * 1000)
+      });
+
+      res.json({
+        success: true,
+        message: '分片上传完成，文件正在转存到云端',
+        path: session.target_path,
+        file_name: session.file_name,
+        file_size: fileSize
+      });
+
+      // 后台异步执行 OSS 上传，不阻塞客户端
+      const bgStorage = storage;
+      storage = null; // 防止 finally 提前关闭
+      (async () => {
+        try {
+          await bgStorage.put(session.temp_file_path, session.target_path);
+          clearOssUsageCache(req.user.id);
+          UploadSessionDB.setStatus(session.session_id, 'completed', {
+            completed: true,
+            expiresAt: buildResumableUploadExpiresAt(60 * 1000)
+          });
+          safeUnlink(session.temp_file_path);
+          await trackFileHashIndexForUpload({
+            userId: req.user.id,
+            storageType: sessionStorageType,
+            fileHash: session.file_hash,
+            fileSize,
+            filePath: session.target_path
+          });
+          console.log(`[分片上传] OSS 后台转存完成: ${session.target_path}`);
+        } catch (bgError) {
+          console.error(`[分片上传] OSS 后台转存失败: ${session.target_path}`, bgError);
+          UploadSessionDB.setStatus(session.session_id, 'failed', {
+            completed: false,
+            expiresAt: buildResumableUploadExpiresAt(60 * 60 * 1000)
+          });
+        } finally {
+          await bgStorage.end().catch(() => {});
+        }
+      })();
+      return;
     }
 
+    // 本地存储：直接同步完成（rename-first 策略已优化，速度很快）
+    await storage.put(session.temp_file_path, session.target_path);
+
     UploadSessionDB.setStatus(session.session_id, 'completed', {
       completed: true,
       expiresAt: buildResumableUploadExpiresAt(60 * 1000)