fix: golangci-lint 修复（gofmt 格式化 + errcheck 返回值检查）

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/backup_service.go

@@ -47,7 +47,7 @@ type BackupS3Config struct {
 	Region          string `json:"region"`   // R2 用 "auto"
 	Bucket          string `json:"bucket"`
 	AccessKeyID     string `json:"access_key_id"`
-	SecretAccessKey string `json:"secret_access_key,omitempty"`
+	SecretAccessKey string `json:"secret_access_key,omitempty"` //nolint:revive // field name follows AWS convention
 	Prefix          string `json:"prefix"`                      // S3 key 前缀，如 "backups/"
 	ForcePathStyle  bool   `json:"force_path_style"`
 }
@@ -454,14 +454,14 @@ func (s *BackupService) RestoreBackup(ctx context.Context, backupID string) erro
 	if err != nil {
 		return fmt.Errorf("S3 download failed: %w", err)
 	}
-	defer result.Body.Close()
+	defer func() { _ = result.Body.Close() }()
 
 	// 解压 gzip
 	gzReader, err := gzip.NewReader(result.Body)
 	if err != nil {
 		return fmt.Errorf("gzip reader: %w", err)
 	}
-	defer gzReader.Close()
+	defer func() { _ = gzReader.Close() }()
 
 	sqlData, err := io.ReadAll(gzReader)
 	if err != nil {