arrstack: raise arrstack-postgres resources after OOMKills (#408)

## Why

Both `arrstack-postgres` replicas were OOMKilled at the 1Gi memory limit on 2026-08-23 07:19, causing a failover to timeline 14, leaving replica-3 unrecoverable via pg_rewind, and blocking the argocd sync wave on Cluster Ready=False.

## Changes

- Raises the CNPG Cluster resources to requests 1Gi memory / 250m CPU and limits 2Gi memory / 1 CPU

**NOTE:** applying this restarts the postgres instances (CNPG rolling restart).

Reviewed-on: #408
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

apps/base/arrstack/postgres/cnpg_cluster.yaml

@@ -147,10 +147,10 @@ spec:
   resources:
     limits:
       cpu: "1"
-      memory: 1Gi
+      memory: 2Gi
     requests:
-      cpu: 50m
-      memory: 512Mi
+      cpu: 250m
+      memory: 1Gi
   smartShutdownTimeout: 180
   startDelay: 3600
   stopDelay: 1800