chore(webapp): consolidate clickhouse read-replica server-changes into one file

Author: ericallam

.server-changes/clickhouse-read-replica.md

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+---
+area: webapp
+type: improvement
+---
+
+Optionally route ClickHouse read traffic to a read replica while writes stay on the primary. Set `CLICKHOUSE_READER_URL` to move all reads, or target the busiest paths with `RUNS_LIST_CLICKHOUSE_URL` (runs list) and `EVENTS_READER_CLICKHOUSE_URL` (traces, spans, logs). All optional; unset keeps current behavior.