Get a page with its legacy blocks

Setting workspaceId moves the page to another space the caller can access — the entire subtree moves with it and the page becomes a root of the target space unless the same request also names a parent inside it. A sidebar drag sends all three move fields at once (space, parent, position), so parentId is validated: - the parent must be a page the caller can see (404 otherwise — never a confirmation that an id exists in another org) - the parent must be in the space the page ends up in, so a page is never nested under a parent that space's readers cannot open (400) - the parent must not be the page itself or one of its own descendants (400) — a cycle detaches the whole branch from every root and it stops appearing in the tree `parentId: ""` clears the parent, putting the page at the top level of its space. `position` is a float, so a page can be dropped between two siblings by writing one row instead of renumbering the list.