修复: 上传操作级重试，解决 Google Drive 等远端临时故障导致自动备份连续失败

问题：rclone 底层重试只覆盖单个 HTTP 请求，但 Google API 的 502/timeout
等临时故障会导致整个上传操作失败，自动触发的备份任务连续失败。

修复：在 provider.Upload 外层增加操作级重试（最多 3 次，指数退避 10s/40s/90s），
每次重试重新打开文件并重建 reader 链。重试过程通过日志流实时反馈。

server/internal/service/backup_execution_service.go

@@ -363,17 +363,24 @@ func (s *BackupExecutionService) executeTask(ctx context.Context, task *model.Ba
 				logger.Warnf("存储目标 %s 创建客户端失败：%v", targetName, resolveErr)
 				return
 			}
+			logger.Infof("开始上传备份到存储目标：%s", targetName)
+			// 上传级重试：最多 3 次，指数退避（10s, 30s, 90s）
+			maxAttempts := 3
+			var lastUploadErr error
+			var hr *hashingReader
+			for attempt := 1; attempt <= maxAttempts; attempt++ {
+				if attempt > 1 {
+					backoff := time.Duration(attempt*attempt) * 10 * time.Second
+					logger.Warnf("存储目标 %s 第 %d 次重试（等待 %v）：%v", targetName, attempt, backoff, lastUploadErr)
+					time.Sleep(backoff)
+				}
 				artifact, openErr := os.Open(finalPath)
 				if openErr != nil {
 					uploadResults[index] = StorageUploadResultItem{StorageTargetID: targetID, StorageTargetName: targetName, Status: "failed", Error: openErr.Error()}
 					logger.Warnf("存储目标 %s 打开备份文件失败：%v", targetName, openErr)
 					return
 				}
-			defer artifact.Close()
+				hr = newHashingReader(artifact)
-			logger.Infof("开始上传备份到存储目标：%s", targetName)
-			// hashingReader: 上传过程中同步计算字节数 + SHA-256，单次读取零额外 I/O
-			hr := newHashingReader(artifact)
-			// progressReader: 包装 hashingReader，通过 LogHub 推送实时上传进度
 				pr := newProgressReader(hr, fileSize, func(bytesRead int64, speedBps float64) {
 					percent := float64(0)
 					if fileSize > 0 {
@@ -387,9 +394,15 @@ func (s *BackupExecutionService) executeTask(ctx context.Context, task *model.Ba
 						TargetName: targetName,
 					})
 				})
-			if uploadErr := provider.Upload(ctx, storagePath, pr, fileSize, map[string]string{"taskId": fmt.Sprintf("%d", task.ID), "recordId": fmt.Sprintf("%d", recordID)}); uploadErr != nil {
+				lastUploadErr = provider.Upload(ctx, storagePath, pr, fileSize, map[string]string{"taskId": fmt.Sprintf("%d", task.ID), "recordId": fmt.Sprintf("%d", recordID)})
-				uploadResults[index] = StorageUploadResultItem{StorageTargetID: targetID, StorageTargetName: targetName, Status: "failed", Error: uploadErr.Error()}
+				artifact.Close()
-				logger.Warnf("存储目标 %s 上传失败：%v", targetName, uploadErr)
+				if lastUploadErr == nil {
+					break
+				}
+			}
+			if lastUploadErr != nil {
+				uploadResults[index] = StorageUploadResultItem{StorageTargetID: targetID, StorageTargetName: targetName, Status: "failed", Error: lastUploadErr.Error()}
+				logger.Warnf("存储目标 %s 上传失败（已重试 %d 次）：%v", targetName, maxAttempts, lastUploadErr)
 				return
 			}
 			// 完整性校验：对比实际传输字节数