Use the traefik-internal ingress class

Both IngressRoutes (server and Grafana) now use traefik-internal, like
the rest of *.k8s.stinnesbeck.com, instead of the traefik-external
placeholder. README and SPECS updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QLJfZQZeAKAwSqViTmLzki

SPECS.md

@@ -41,8 +41,7 @@ car ──live telemetry───▶ car.k8s.stinnesbeck.com (Go) ──▶ ABRP
 - [x] Car app 2.4.0 (pairing by QR): unit tests pass; **not yet tried on the car**.
 - [x] Image `mg4-server:2.1.0` (with pairing) pushed to git.stinnesbeck.com (public; pinned by
       digest `sha256:e5f6e2d2…` in `k8s/kustomization.yaml`).
-- [ ] Check in `k8s/ingress.yaml`: the Traefik ingress class (placeholder `traefik-external`) and
-      Grafana's host (placeholder `car-grafana.k8s.stinnesbeck.com`).
+- [ ] Check Grafana's host in `k8s/ingress.yaml` (placeholder `car-grafana.k8s.stinnesbeck.com`).
 - [ ] First pairing and drive with 2.4.0: telemetry arrives in InfluxDB, Grafana and ABRP.
 
 ## To do