Resolve experiment context for a batch of session recordings.

Batch variant of session_context, used to prefetch the replay player's experiments box for a whole recordings list in one request. POST because the id list doesn't fit a query string; the endpoint only reads. Already-computed sessions are served from (and cold ones written to) the same short-lived per-viewer cache the single-session endpoint uses, so opening any prefetched recording renders its context instantly. Sessions whose recording metadata doesn't exist yet are omitted from the response, as are recordings the caller can't access and sessions beyond the batch's recording-day budget (each distinct recording day costs its own set of ClickHouse scans, so only the most recent days are computed per request).