Upsert semantics (v1)

POST inserts or updates each (source, alternative) pair; it never removes links. Re-sending the same pair is idempotent. There is no delete endpoint yet — omitting a previously sent alternative leaves it in place. A row whose source or alternative article_number does not exist in your catalogue is skipped (counted in the job's skipped_count), not an error. If any row is structurally invalid (missing a required field, a negative order) the whole batch is rejected and the per-row detail appears in GET /jobs/{job_id}.