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/kustomization.yaml

@@ -4,8 +4,10 @@ kind: Kustomization
 
 resources:
   - namespace.yaml
-  - pvc.yaml
+  - cnpg_cluster.yaml
+  - cnpg_pooler.yaml
   - deployment.yaml
+  - pdb.yaml
   - service.yaml
   - gateway.yaml
   - httproute.yaml