fix: 恢复腾讯云账单同步并完善成本预测

- 接入腾讯云单日账单、全局限频和指数退避

- 修正月末费用预测并增加账期提示和费用归因

- 补充腾讯云限频及账单归因回归测试

backend/router/routes.go

@@ -323,6 +323,7 @@ func Register(r *gin.Engine, db *gorm.DB, log *zap.Logger, cfg *config.Config, c
 		billings.POST("/batch", billingHandler.BatchCreateBillings)
 		billings.DELETE("/:id", billingHandler.DeleteBilling)
 		billings.GET("/stats", billingHandler.GetBillingStats)
+		billings.GET("/cost-analysis", billingHandler.GetBillingCostAnalysis)
 		billings.GET("/providers", billingHandler.GetProviderOptions)
 		billings.POST("/sync", billingHandler.SyncYesterdayBillings)
 	}