Design a Collaborative Todo List

Design a collaborative todo-list service. The service should allow users to create shared lists, add and edit todo items, assign owners, set due dates, reorder items, mark items complete, and see other collaborators updates in near real time. Clarify and design for: - Multiple users editing the same list concurrently. - Authentication and per-list permissions. - Real-time synchronization across web and mobile clients. - Offline or intermittent connectivity support if time permits. - Notifications for assignment, comments, due dates, or completed tasks. - Reliability, observability, and data consistency trade-offs. Assume a consumer or team-product scale, such as millions of users and tens of thousands of concurrent collaborators.