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/httproute.yaml

@@ -47,3 +47,7 @@ spec:
         - path:
             type: PathPrefix
             value: /
+      sessionPersistence:
+        type: Cookie
+        cookieName: open-webui-backend
+        absoluteTimeout: 24h0m0s