## Summary
When a background worker registers, the engine resolves runs that were
queued before the worker was ready (status `PENDING_VERSION`). That
lookup used to scan a Postgres status index on `TaskRun`. Move it to
ClickHouse: query candidate run ids from `task_runs_v2`, then refetch
the actual rows from Postgres by primary key with a `status =
'PENDING_VERSION'` guard for idempotency.
## Design
The lookup is a pluggable interface on the run engine
(`PendingVersionRunIdLookup`). The webapp wires a ClickHouse-backed
implementation through the org-scoped `clickhouseFactory` using a new
`"engine"` client type, configured by `RUN_ENGINE_CLICKHOUSE_*` env
vars. The URL falls back to `CLICKHOUSE_URL` when unset, so self-hosted
deployments don't need new config to keep working.
When the lookup returns no candidates, one bounded retry is scheduled
~5s later to cover ClickHouse replication lag against `task_runs_v2`.
The Postgres status guard on both the candidate refetch and the inner
`updateMany` prevents double-promotion when a retry races with a
concurrent deploy.
Tests cover three existing PENDING_VERSION cases via a small
Postgres-backed test adapter; new ClickHouse-backed integration tests
will follow.
Eric Allam
61ca40b4b1