Design Shared Links with Expiration

Design a highly-available service that lets Roblox developers create time-limited public download links for game assets (maps, meshes, audio). A link is a short, opaque token that resolves to an asset URI. Links must auto-expire at a developer-chosen time (1 min → 30 days) and immediately become unusable thereafter. The system must serve 200 k read QPS globally with p99 < 50 ms, accept 5 k create QPS, and work through regional outages. Expiration must be memory-bounded and shardable; heap-based TTL is not enough at scale. Outline APIs, data model, caching, TTL enforcement, cleanup, and multi-region failover.