A retail company is designing a new order-processing backend on Azure. The workload consists of short, stateless functions triggered by messages arriving on an Azure Service Bus queue. Message volume is highly variable — near zero overnight and spiking to thousands per minute during flash sales. The development team wants the least operational overhead, automatic scaling driven directly by queue depth, and the ability to scale to zero to minimize cost during idle periods. They do not want to manage or configure any container orchestration or virtual machines. Which compute service should you recommend?