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/.helmignore

@@ -0,0 +1,17 @@
+.DS_Store
+.git/
+.gitignore
+.bzr/
+.bzrignore
+.hg/
+.hgignore
+.svn/
+*.swp
+*.bak
+*.tmp
+*.orig
+*~
+.project
+.idea/
+*.tmproj
+.vscode/