feat(artifactapi): migrate Ingress to Gateway API (#129)

## 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

## Notes

The original Ingress had nginx-specific annotations (`proxy-body-size: 10g`, `proxy-read-timeout: 600`) which are not portable to Gateway API. These can be re-introduced via a Traefik `Middleware` CRD if needed.

## Test plan

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

Reviewed-on: #129

apps/base/artifactapi/gateway.yaml

@@ -0,0 +1,29 @@
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: Gateway
+metadata:
+  labels:
+    traefik.io/instance: internal
+  annotations:
+    cert-manager.io/cluster-issuer: vault-issuer
+    cert-manager.io/common-name: artifactapi.k8s.syd1.au.unkin.net
+    cert-manager.io/private-key-size: "4096"
+    external-dns.alpha.kubernetes.io/hostname: artifactapi.k8s.syd1.au.unkin.net
+    external-dns.alpha.kubernetes.io/target: 198.18.200.4
+  name: artifactapi
+  namespace: artifactapi
+spec:
+  gatewayClassName: traefik-internal
+  listeners:
+    - allowedRoutes:
+        namespaces:
+          from: Same
+      hostname: artifactapi.k8s.syd1.au.unkin.net
+      name: https
+      port: 443
+      protocol: HTTPS
+      tls:
+        certificateRefs:
+          - kind: Secret
+            name: artifactapi-tls
+        mode: Terminate