Returns a single deployment by id. The response includes status (deployment_status, files_status, code_status), commit metadata, timing, and any failure reason — sufficient to monitor progress without needing the dedicated logs endpoint for most use cases. For deployments created by an environment promote or rollback, the response also carries the saga columns (env_version, source_env_id, source_deployment_id, aborted_by_deployment_id, note, actor_id, environment_name, environment_epoch); these are null on legacy build-pipeline rows.