integ: pin the langfuse images

The 3 tag was repointed upstream on 2026-09-01 at 14:50Z to an image
whose clickhouse migration wants 25.x text-index syntax, which the
pinned clickhouse 24.12 refuses with code 80. langfuse-web then exits
on its migration and never binds 3000, so integ-observability went red
on main and on every open PR with no code change.