Dashboards in Grafana; clean up after the Next.js app

- k8s/grafana.yaml: Grafana 13.2.3 with a 1Gi PVC, sign-up and
  anonymous access off. The InfluxDB datasource (URL and token from the
  environment) and three dashboards — Live, Consumption, Charging — are
  provisioned from k8s/grafana/ via kustomize-generated ConfigMaps.
- server.yaml for the Go server (port 8080, UID 65532, smaller limits),
  image tag 2.0.0. The NetworkPolicy now admits Grafana to InfluxDB.
- ingress.yaml: IngressRoutes for car.stinnesbeck.com and Grafana
  (placeholder host car-grafana.stinnesbeck.com) with tls: {}, using
  Traefik's default certificate; the cert-manager Certificate is gone.
- secrets.example.yaml: drops the dashboard secrets, adds grafana-admin.
- AGENTS.md loses its Next.js block; SPECS.md describes the new setup.

The removal of web/ (the Next.js app) and of the stale .next/ cache
upstream had committed already landed in 51e6f6f, which was staged
together with the Go server by mistake.

Checked end to end in podman (Go server + InfluxDB 2.7.12 + Grafana
13.2.3): all 25 dashboard queries return the expected data, and the
dashboards were rendered with headless Chromium. Manifests validated
offline with kustomize and kubeconform. Nothing applied to a cluster.

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

k8s/influxdb.yaml

@@ -110,11 +110,12 @@ spec:
       port: 8086
       targetPort: http
 ---
-# Only the server may reach InfluxDB. Has no effect on a CNI without NetworkPolicy support.
+# Only the server and Grafana may reach InfluxDB. Has no effect on a CNI without
+# NetworkPolicy support.
 apiVersion: networking.k8s.io/v1
 kind: NetworkPolicy
 metadata:
-  name: influxdb-from-server-only
+  name: influxdb-from-server-and-grafana
   namespace: mg4
 spec:
   podSelector:
@@ -126,5 +127,8 @@ spec:
         - podSelector:
             matchLabels:
               app.kubernetes.io/name: mg4-server
+        - podSelector:
+            matchLabels:
+              app.kubernetes.io/name: grafana
       ports:
         - port: 8086