feat(open-webui): HA deployment with CNPG, PDB, and session persistence

- Switch from SQLite/PVC to CNPG PostgreSQL (3 instances, low-resource)
  with a transaction-mode PgBouncer pooler (2 instances)
- Raise open-webui replicas to 3 with priorityClassName: power
- Add PodDisruptionBudget (minAvailable: 1)
- Add Gateway API sessionPersistence (cookie) on the HTTPS HTTPRoute
  so WebSocket connections stick to the same backend pod
- Add postgres-credentials VaultStaticSecret; DATABASE_URL must be
  added to kv/kubernetes/namespace/open-webui/default/open-webui-credentials

apps/base/open-webui/deployment.yaml

@@ -5,12 +5,10 @@ metadata:
   name: open-webui
   namespace: open-webui
 spec:
-  replicas: 1
+  replicas: 3
   selector:
     matchLabels:
       app: open-webui
-  strategy:
-    type: Recreate
   template:
     metadata:
       annotations:
@@ -18,6 +16,7 @@ spec:
       labels:
         app: open-webui
     spec:
+      priorityClassName: power
       containers:
         - name: open-webui
           image: ghcr.io/open-webui/open-webui:main
@@ -59,11 +58,4 @@ spec:
             requests:
               cpu: 250m
               memory: 512Mi
-          volumeMounts:
-            - mountPath: /app/backend/data
-              name: data
       restartPolicy: Always
-      volumes:
-        - name: data
-          persistentVolumeClaim:
-            claimName: open-webui-data