You are designing a Cosmos DB container for a product catalog used by an e-commerce site. The application performs the following operations: (1) 95% of requests are individual product lookups by productId when a customer clicks a product page, and (2) 5% are admin queries that list all products within a single categoryId. There are roughly 40 balanced categories and millions of products. You want to minimize RU cost for the dominant workload while keeping writes well distributed. Which partition key should you choose?