chore(k8s): pin postgres to 18.6-alpine

The floating 18-alpine tag with IfNotPresent kept an old 18.x image
cached on the nodes, so minor releases never reached the cluster.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

deploy/k8s/postgres/statefulset.yaml

@@ -15,7 +15,7 @@ spec:
     spec:
       containers:
         - name: postgres
-          image: postgres:18-alpine
+          image: postgres:18.6-alpine
           ports:
             - containerPort: 5432
           envFrom: