fix: 修复批量更新凭证明细与缓存TTL抖动

- BatchUpdateCredentials 返回 success/failed/results 及 success_ids/failed_ids

- billing jitteredTTL 改为只减不增，确保TTL不超上界

- crypto/rand 失败时随机ID降级避免 panic

- OpenAI SelectAccount 失败日志去重并补充字段

- 修复两处类型断言以通过 errcheck

backend/internal/handler/openai_gateway_handler.go

@@ -219,9 +219,8 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 		log.Printf("[OpenAI Handler] Selecting account: groupID=%v model=%s", apiKey.GroupID, reqModel)
 		selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), apiKey.GroupID, sessionHash, reqModel, failedAccountIDs)
 		if err != nil {
-			log.Printf("[OpenAI Handler] SelectAccount failed: %v", err)
+			log.Printf("[OpenAI Handler] SelectAccount failed: groupID=%v model=%s tried=%d err=%v", apiKey.GroupID, reqModel, len(failedAccountIDs), err)
 			if len(failedAccountIDs) == 0 {
-				log.Printf("[OpenAI Gateway] SelectAccount failed: %v", err)
 				h.handleStreamingAwareError(c, http.StatusServiceUnavailable, "api_error", "Service temporarily unavailable", streamStarted)
 				return
 			}