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/pvc.yaml

@@ -0,0 +1,13 @@
+---
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: open-webui-data
+  namespace: open-webui
+spec:
+  accessModes:
+    - ReadWriteOnce
+  resources:
+    requests:
+      storage: 10Gi
+  storageClassName: cephrbd-fast-delete