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/values.yaml

@@ -0,0 +1,16 @@
+networking:
+  hostname: "terdut.example.com"
+  servicePort: 8080
+
+image:
+  repository: ghcr.io/yeniklas/terdut-server
+  tag: "latest"
+  pullPolicy: IfNotPresent
+
+storage:
+  size: 1Gi
+  storageClass: synology-iscsi
+
+service:
+  type: ClusterIP
+  port: 8080