## Summary
Adds optional configuration to send ClickHouse read traffic to a
separate instance (for example a read replica) while writes stay on the
primary `CLICKHOUSE_URL`. This lets operators offload read load (runs
list, traces, logs, queries) from the cluster that handles inserts.
Fully backwards compatible: with nothing new set, every client resolves
to `CLICKHOUSE_URL` exactly as before.
## What it adds
- `CLICKHOUSE_READER_URL` (optional): a single reader endpoint that the
read-only clients fall back to. Read clients resolve `<own URL> ??
CLICKHOUSE_READER_URL ?? CLICKHOUSE_URL`. The task-events client (which
both inserts events and reads traces, spans, and logs) is built as a
reader/writer pair so queries use the reader while inserts stay on
`CLICKHOUSE_URL`.
- `RUNS_LIST_CLICKHOUSE_URL` (optional): a dedicated client for the runs
list (dashboard list, runs list API, live reload, child-status counts),
so the highest-traffic read path can target its own instance.
## Safety
Only read-only clients fall back to the reader: logs, query, admin, runs
list, the pending-version lookup, and the realtime run-id resolver. The
query page is constrained to read-only (the TSQL parser rejects anything
that is not a `SELECT`, and a `readonly` setting is applied). The
task-events client routes inserts to the writer and queries to the
reader per method, so a write can never reach the reader. Pure-write
clients (event inserts, replication) always use `CLICKHOUSE_URL`.
Note: this PR targets a baseline branch rather than `main` so the diff
stays scoped to the read-replica changes. It will be retargeted to
`main` before merge.
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Co-authored-by: Eric Allam <eallam@icloud.com>
nicktrn
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