Get a pipeline element's job_config

Return one element's job_config, hydrated lazily on expand.

The bulk /elements response projects the bigger keys (parameters, optimizer, cost) out of every element so the details page loads fast; this fills them back in for the one element a drawer is showing. Returns {} for an element that does not exist or is not visible to the caller.

Note this is the DB row, not the storage copy — the two differ only by objectives, the multi-MB serialized measurement payload the worker reads directly and no client needs.