fix(helm): use dedicated pingUrl for external ClickHouse readiness check

Environment: self-hosted via k8s and ArgoCD

When clickhouse.mode is “external”, the chart’s init container uses clickhouse.external.url as the readiness check URL. However, that URL typically contains the database path (e.g. http://user@host:8123/db), and ClickHouse returns HTTP 404 for any path other than /ping. The migration job’s init container therefore never becomes ready, causing deployments to time out.

Add an optional clickhouse.external.pingUrl value. When set, it is used for the readiness check instead of the database URL. When not set, the existing behaviour is preserved (external.url is used as-is).