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/values.yaml

@@ -4,7 +4,7 @@ namespace: sqs
 nodePool:
   replicas: 3
   storage:
-    class: longhorn-kafka
+    class: longhorn
     # Longhorn's per-node scheduling budget on the current 2-node cluster has
     # only ~36Gi of headroom left (other PVCs already reserve the rest), and
     # each node hosts one replica of all 3 broker volumes -- so 3 * sizeGi