The default "longhorn" StorageClass requests 3 replicas, but the homelab
cluster currently has only 2 schedulable nodes, so the 3rd replica could
never be scheduled and volumes stayed permanently degraded. Adds
longhorn-kafka (numberOfReplicas: 2) and reduces broker PVC size so 3 broker
volumes' replicas fit within each node's remaining Longhorn scheduling
headroom. Revert to "longhorn" once a 3rd node joins.