fix(storage): consolidate longhorn-kafka → unified longhorn StorageClass

Removes duplicate longhorn-kafka StorageClass managed by Kafka chart.
All applications now use single 'longhorn' StorageClass (3 replicas, Immediate binding).

Changes:
- Kafka chart: use 'longhorn' instead of 'longhorn-kafka'
- Delete Kafka StorageClass template (no longer needed)
- Update longhorn-storageclass.yaml to match deployed config (Immediate, not WaitForFirstConsumer)

Existing Kafka PVCs remain bound to old longhorn-kafka StorageClass (safe - no data loss).
New PVCs will use unified 'longhorn' StorageClass.

k8s/applications/sqs/charts/kafka-cluster/templates/storageclass.yaml

@@ -1,19 +0,0 @@
-{{- if eq .Values.nodePool.storage.class "longhorn-kafka" }}
-# The default "longhorn" StorageClass requests 3 replicas across 3 zone-labeled
-# nodes (az-a/az-b/az-c). With Longhorn's zone-aware anti-affinity, replicas
-# spread 1-per-zone for durability.
-apiVersion: storage.k8s.io/v1
-kind: StorageClass
-metadata:
-  name: longhorn-kafka
-provisioner: driver.longhorn.io
-allowVolumeExpansion: true
-reclaimPolicy: Delete
-volumeBindingMode: Immediate
-parameters:
-  numberOfReplicas: "3"
-  staleReplicaTimeout: "30"
-  fromBackup: ""
-  fsType: "ext4"
-  dataLocality: "disabled"
-{{- end }}