Autosave the Overview dashboard the caller is editing, as a private draft

Sent when a user leaves the Overview page in edit mode with unsaved changes (often with fetch keepalive). Creates a dashboard version with status preview and sourceType autosave, authored by the caller. It never changes which version is published: GET /dashboard/layouts/latest and /overview/dashboard-spec/latest are unaffected. In the same transaction the caller's autosaves in the organization beyond the newest 5 are deleted permanently (so are the caller's autosaves already deleted from history). Autosaves are visible only to their author in the dashboard history (list, detail, delete, apply); applying one publishes a copy as a new manual version, like applying any other non-AI version. An optional id naming the version the editor started from is stored as the draft's parent. Body limit 2 MiB. Not idempotent. Requires org.edit_dashboard; scoped by X-Greentally-Organization-Id like the other Overview routes.