feat(schema): add WorkerDeployment.externalId and task_runs_v2.external_deployment_id (#4661)

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).