Draw + claim the oldest review_ready CASE for this reviewer.

Returns the case with BOTH labels side by side, blinded, in an A/B order seeded on (task_id, reviewer_id) — stable across reloads so a refresh does not swap the columns, and uncorrelated with submission order so position never leaks who went first.

The store query already excludes cases this reviewer labelled or reviewed; the claim is compare-and-set on the TASK, so two reviewers drawing concurrently can never hold the same pair.

PREVIEW (§4.1) is a different draw, not a flag on this one: no claim, no session, and a draw token the submit path refuses. Restricted to operators — a real reviewer asking for a preview would be asking to work for free, and a labeler asking for one would be asking to read a pair they were excluded from. An operator whose tier does not carry review is FORCED into it, which is the whole point: clicking through the reviewer surface must not grade a physician's work by accident.