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/grafana/datasources.yaml

@@ -0,0 +1,17 @@
+# Provisioned InfluxDB datasource. URL and token come from the Grafana container's
+# environment (INFLUX_URL, INFLUX_TOKEN), so no credential is stored in this file.
+apiVersion: 1
+datasources:
+  - name: InfluxDB (mg4)
+    uid: mg4-influxdb
+    type: influxdb
+    access: proxy
+    url: $INFLUX_URL
+    isDefault: true
+    editable: false
+    jsonData:
+      version: Flux
+      organization: mg4
+      defaultBucket: mg4
+    secureJsonData:
+      token: $INFLUX_TOKEN