You are designing a Cosmos DB account for an internal developer sandbox and a new microservice. The sandbox receives sporadic, unpredictable traffic with long idle periods and rarely exceeds 4,000 RU/s during brief bursts; its total data footprint stays under 20 GB. Cost efficiency is the top priority, and you want to avoid paying for provisioned throughput during the many hours the workload is idle. Which capacity mode should you choose for this container?