feat(postgres): explicitly define affinity

- use topology.kubernetes.io/zone instead of kubernetes.io/hostname as topology key

manifests/postgres/postgres.yaml

@@ -34,3 +34,7 @@ spec:
   replicationSlots:
     highAvailability:
       enabled: true
+  affinity:
+    enablePodAntiAffinity: true
+    topologyKey: topology.kubernetes.io/zone # default: kubernetes.io/hostname
+    podAntiAffinityType: preferred