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/gateway.yaml

@@ -0,0 +1,37 @@
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: Gateway
+metadata:
+  labels:
+    traefik.io/instance: external
+  annotations:
+    cert-manager.io/cluster-issuer: vault-issuer
+    cert-manager.io/common-name: chat.k8s.syd1.au.unkin.net
+    cert-manager.io/private-key-size: "4096"
+    external-dns.alpha.kubernetes.io/hostname: chat.k8s.syd1.au.unkin.net
+    external-dns.alpha.kubernetes.io/target: 198.18.200.0
+  name: open-webui
+  namespace: open-webui
+spec:
+  gatewayClassName: traefik-external
+  listeners:
+    - allowedRoutes:
+        namespaces:
+          from: Same
+      hostname: chat.k8s.syd1.au.unkin.net
+      name: http
+      port: 80
+      protocol: HTTP
+    - allowedRoutes:
+        namespaces:
+          from: Same
+      hostname: chat.k8s.syd1.au.unkin.net
+      name: https
+      port: 443
+      protocol: HTTPS
+      tls:
+        certificateRefs:
+          - group: ""
+            kind: Secret
+            name: open-webui-tls
+        mode: Terminate