fix(monitoring): Prometheus memory limit 1Gi -> 2Gi (OOMKilled on compaction)

prometheus-...-0 was OOMKilled (exit 137) every ~4min during TSDB compaction/checkpoint
(261 restarts/37h). Raise limit to 2Gi + request to 512Mi; nodes have headroom.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

platform/monitoring/kube-prometheus-stack.yaml

@@ -56,9 +56,10 @@ spec:
         resources:
           requests:
             cpu: 100m
-            memory: 384Mi
+            memory: 512Mi
           limits:
-            memory: 1Gi
+            memory: 2Gi                        # 1Gi OOMKilled (exit 137) during TSDB compaction/checkpoint
+                                               # spikes; 2Gi holds. Nodes have headroom (~2Gi free each).
         storageSpec:
           volumeClaimTemplate:
             spec: