fix(spark): increase worker cores to 4 to allow concurrent jobs

With SPARK_WORKER_CORES=1 and 2 workers the cluster has 2 total cores.
LiveDashboard holds both — batch jobs wait indefinitely. 4 cores per
worker gives 8 total, enough to run streaming + batch concurrently.

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

spark-cluster/base/workers.yaml

@@ -25,7 +25,7 @@ spec:
             - spark://spark-master:7077
           env:
             - name: SPARK_WORKER_CORES
-              value: "1"
+              value: "4"
             - name: SPARK_WORKER_MEMORY
               value: "512m"
           ports: