feat(open-webui): deploy Open WebUI with litellm backend

Deploys Open WebUI (chat.k8s.syd1.au.unkin.net) into the open-webui
namespace via the aitooling ArgoCD project. Uses SQLite with a 10Gi
cephrbd PVC for persistence, routes model requests to the existing
litellm deployment, and exposes the UI through the traefik-external
gateway. Credentials (OPENAI_API_KEY, WEBUI_SECRET_KEY) are injected
via VaultStaticSecret from kv/kubernetes/namespace/open-webui/default.

Closes #155

apps/base/open-webui/vaultstaticsecret.yaml

@@ -0,0 +1,17 @@
+---
+apiVersion: secrets.hashicorp.com/v1beta1
+kind: VaultStaticSecret
+metadata:
+  name: open-webui-credentials
+  namespace: open-webui
+spec:
+  destination:
+    create: true
+    name: open-webui-credentials
+    overwrite: true
+  hmacSecretData: true
+  mount: kv
+  path: kubernetes/namespace/open-webui/default/open-webui-credentials
+  refreshAfter: 5m
+  type: kv-v2
+  vaultAuthRef: default