fix(webapp): separate the webhook replication database connection

Add `WEBHOOK_DELIVERIES_REPLICATION_DATABASE_URL` so webhook delivery
replication to ClickHouse can use a direct PostgreSQL connection with
its own credentials while application queries use a pooler. When unset,
replication continues to use `WEBHOOK_DATABASE_URL`, falling back to
`DATABASE_URL`.

Mono-RevId: 9991ab7c7dd7df8f00e3a93a94ca2b8ad02bf4cf