ETS-backed in-memory task backend.

The default implementation keeps one private ETS table for task bodies and dedicated ordered indexes for listing and expiry:

tasks              -> task_id => task
created_index      -> {-submitted_at, task_id} => session_id
session_created    -> {session_id, -submitted_at, task_id} => true
expiry_index       -> {expires_at, task_id} => true

The important part is not the exact table layout; it is that the runtime can paginate and prune without re-sorting the full task set on every request.