Resolve the most recent pipeline job_id for a Mode 2 society.

The Mode 2 flow goes:

1. UI → POST /v1/societies/create-from-integrations → returns society_uuid + society_key synchronously, but the pipeline job_id doesn't exist yet (the orchestrator hasn't called digital-twin-api /pipelines/full yet).
2. Orchestrator (background) does Bronze pushes, then triggers /pipelines/full, gets job_id back, stashes it on every society_source_connections row for that society.
3. UI polls THIS endpoint with (market, name) until job_id appears (could take a few seconds for fast Mode 2 / many minutes for heavy Bronze pushes), then switches to polling /api/societies/pipelines/jobs/{job_id} for live progress.

Returns {job_id: <uuid>} once stashed, {job_id: null} while the orchestrator is still pre-pipeline. Mode 1 societies have no society_source_connections row, so this endpoint will keep returning null for them — Mode 1 callers receive job_id directly from /api/societies/pipelines/full and don't need to dereference.