fix(traefik): add instance labels to GatewayClasses (#137)

## Problem

GatewayClasses were `Unknown` even after controllerName was fixed. The `kubernetesGateway` `labelSelector` applies to all watched resources, including GatewayClasses themselves. Since neither GatewayClass had a `traefik.io/instance` label, both Traefik instances filtered them out and never accepted them.

## Fix

- `gatewayclass-internal.yaml`: add `traefik.io/instance: internal`
- `gatewayclass-external.yaml`: add `traefik.io/instance: external`

## Test plan

- [ ] `kubectl get gatewayclass` shows both as `Accepted: True`

Reviewed-on: #137

apps/base/traefik-system/gatewayclass-external.yaml

@@ -2,6 +2,8 @@
 apiVersion: gateway.networking.k8s.io/v1
 kind: GatewayClass
 metadata:
+  labels:
+    traefik.io/instance: external
   name: traefik-external
 spec:
   controllerName: traefik.io/gateway-controller