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Migrations only, no code reads them yet. Postgres: nullable non-unique externalId on WorkerDeployment plus a CONCURRENTLY-built (environmentId, externalId) index in its own migration file. ClickHouse: external_deployment_id String DEFAULT '' on task_runs_v2 (plain String, not LowCardinality - commit SHAs are high-cardinality). Part of task run version skew protection (TRI-12998).