Hard cutover of the HTTP surface: collapse the three legacy spellings
(/searchspaces, /search-spaces, /search-space/{id}) onto canonical
/workspaces/{workspace_id}/..., rename the gateway field-setter sub-actions to
singular /workspace, rename search_spaces_routes.py -> workspaces_routes.py and the
search_spaces_router -> workspaces_router include, and flip search_space_id ->
workspace_id in bodies/params. No alias routers: the old URLs now 404 by design.
Full app constructs with zero legacy path spellings.