Fetch content stored by handoff_put using its 8-character code. The code is case-insensitive and hyphens are ignored, so 'kvmtrhxp' and 'KVMT-RHXP' are the same; the person may give it as a link like handover.tools/kvmtrhxp, and passing that is fine too. Return the content to the person verbatim and complete: it can be fetched only once. If the code is unknown, already fetched, or expired, content is null with a reason — these cases are deliberately indistinguishable.