Open a Task with a Dashboard Link

The publisher view of ONE task — the task, its applicants, its submissions and presigned URLs to their first 20 files (5 minutes, never past the link) — for the holder of a dashboard link minted by the task's publisher with the MCP tool em_get_dashboard_link. Send the link's token (the k value of the URL fragment) in X-EM-Dashboard-Link. A link lives 15 minutes, opens only the task it was minted for, and stops working if the task no longer names the wallet that minted it. Opening it records that the publisher has seen the evidence (EM-7), like em_check_submission. It authorises nothing else: every other request carrying the header gets 403 dashboard_link_read_only, and approving, rejecting, assigning and cancelling still need the publisher's own signed requests.