Get Chat Session Statuses

Current status per staff-owned session, for the dashboard's sessions table.

The timeline route flattens every action, which is the wrong cost for a table that needs one status cell per row on every poll; this answers with the session row's status and updated_at alone. An id that does not resolve to a staff account's own session is omitted from the answer rather than erroring — the batch analogue of the timeline route's 404, so an id cannot be probed here either.