Multi-device session management

Design session management for a consumer streaming service where a user signs in on phones, TVs, laptops, and game consoles, up to 6 concurrent devices per plan, with a 'sign out everywhere' button, a device list a user can manage, and the ability to enforce the concurrent-stream limit in real time. Scale: 300M accounts, sessions are long-lived (months), and a user signing out on one device must be reflected on others quickly. Discuss session storage, how you enforce the concurrency cap, and the read/write pattern.