Answer
The stuck entries live in XLR_REPORT_RENDERING_INFO, and XLR_REPORT_RENDERING_INFO_API.REMOVE_INSTANCE(report_id, result_key) is the API that removes a specific stuck instance. A small script that queries that table for rows still marked 'Rendering in Progress' past a reasonable age threshold (e.g. more than 3 hours old) and calls REMOVE_INSTANCE on each is a practical way to clear a backlog of genuinely-stuck reports and unblock the queue for everything else.