Add Helm chart and chart-release workflow

charts/terdut-server/ — Helm chart for Kubernetes deployment:
  - Deployment (replicas=1, /healthz probes, TERDUT_DB_PATH=/data/terdut.db)
  - Service (ClusterIP :8080)
  - PVC (1Gi, synology-iscsi) mounted at /data
  - HTTPRoute via envoy-main gateway

.github/workflows/chart-release.yml — packages and publishes the chart to
gh-pages branch on any push to main that touches charts/; repo URL will be
https://yeniklas.github.io/terdut-server once the repo is made public

charts/terdut-server/Chart.yaml

@@ -0,0 +1,6 @@
+apiVersion: v2
+name: terdut-server
+description: A Helm chart for Terminal Duty — on-call alert management server
+type: application
+version: 0.1.0
+appVersion: "latest"