feat(spark): deploy persistent Spark cluster with Web UI

- Add spark-cluster/base with Bitnami Spark chart 9.2.10 (master +
  2 workers), spark-events PVC (RWO/local-path), and Ingress resources
  for spark.nowchess.janis-eccarius.de (master UI) and
  spark-history.nowchess.janis-eccarius.de (history placeholder)
- Add spark-cluster/eu-central-1-prod overlay and ArgoCD Application
  (automated prune + selfHeal)
- Migrate all spark-analytics CronJobs and streaming Deployment from
  K8s-native mode (k8s://...) to Spark standalone (spark://spark-master:7077);
  drop kubernetes-specific --conf flags; add eventLog confs; mount
  spark-events PVC on all job pods

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spark-cluster/base/pvc.yaml

@@ -0,0 +1,18 @@
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: spark-events
+  labels:
+    app.kubernetes.io/name: spark-cluster
+    app.kubernetes.io/part-of: nowchess
+spec:
+  # ReadWriteOnce for k3d/local-path compatibility.
+  # NOTE: k3d uses the local-path provisioner which only supports RWO.
+  # For production with multiple concurrent writers (workers + history server),
+  # replace with a RWX-capable storage class such as NFS or Longhorn and
+  # change accessModes to [ReadWriteMany].
+  accessModes:
+    - ReadWriteOnce
+  resources:
+    requests:
+      storage: 5Gi