feat(litellm): migrate Ingress to Gateway API (#134)

## Summary

- Replace `Ingress` (nginx) with `Gateway` + `HTTPRoute` using `traefik-internal` GatewayClass
- TLS terminated at the Gateway listener; cert-manager provisions the certificate via `vault-issuer`
- external-dns annotations moved to the Gateway

## Test plan

- [ ] ArgoCD syncs the litellm app cleanly
- [ ] cert-manager issues the `litellm-tls` certificate
- [ ] external-dns creates the DNS record
- [ ] `https://litellm.k8s.syd1.au.unkin.net` is reachable

Reviewed-on: #134

apps/base/litellm/httproute.yaml

@@ -0,0 +1,20 @@
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata:
+  name: litellm
+  namespace: litellm
+spec:
+  hostnames:
+    - litellm.k8s.syd1.au.unkin.net
+  parentRefs:
+    - name: litellm
+      sectionName: https
+  rules:
+    - backendRefs:
+        - name: litellm
+          port: 4000
+      matches:
+        - path:
+            type: PathPrefix
+            value: /