Task manager with TTL expiry and time-travel snapshots

A four-level task manager: per-user task lists first, then TTL-based auto-deletion, then a snapshot read that returns a user's tasks as they stood at any past timestamp.

• Level 1: add_task(ts, user, name, priority) returns "task1", ...; get_task, update_task, delete_task.
• Level 2: list_tasks(ts, user): live task names, highest priority first, ties in creation order.
• Level 3: add_task_with_ttl(ts, user, name, priority, ttl): alive for ts <= t < ts + ttl.
• Level 4: tasks_at(ts, user, at): what list_tasks would have returned at the past time at.

class TaskManager:
    def add_task(self, ts, user, name, priority): ...
    def get_task(self, ts, task_id): ...
    def update_task(self, ts, task_id, name, priority): ...
    def delete_task(self, ts, task_id): ...

    def list_tasks(self, ts, user): ...

    def add_task_with_ttl(self, ts, user, name, priority, ttl): ...

    def tasks_at(self, ts, user, at): ...