Return resources shared by the graph and dcode operation routes.

Builds once and caches. A construction failure is converted into a startup-error marker (scraped by the parent app process) before sys.exit(1), which is right for the langgraph.json graph factory at startup. Callers in request scope must contain that exit -- SystemExit is a BaseException -- as offload_api._execute_offload does, mapping it to a 503 rather than killing the server mid-request.