refactor: 提取 service 层 best-effort 超时常量

- 新增 defaultBETimeout = 5 * time.Second
- 替换 auth/auth_runtime/password_reset/user_service/webhook 中 6 处硬编码 5*time.Second

internal/service/auth_runtime.go

@@ -95,7 +95,7 @@ func (s *AuthService) bestEffortUpdateLastLogin(ctx context.Context, userID int6
 				log.Printf("auth: update last login panic recovered, source=%s user_id=%d err=%v", source, userID, r)
 			}
 		}()
-		bgCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		bgCtx, cancel := context.WithTimeout(context.Background(), defaultBETimeout)
 		defer cancel()
 		if err := s.userRepo.UpdateLastLogin(bgCtx, userID, ip); err != nil {
 			log.Printf("auth: update last login failed, source=%s user_id=%d ip=%s err=%v", source, userID, ip, err)