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/templates/_helpers.tpl

@@ -0,0 +1,34 @@
+{{- define "terdut-server.name" -}}
+{{- default .Chart.Name .Values.nameOverride | trunc 63 | trimSuffix "-" }}
+{{- end }}
+
+{{- define "terdut-server.fullname" -}}
+{{- if .Values.fullnameOverride }}
+{{- .Values.fullnameOverride | trunc 63 | trimSuffix "-" }}
+{{- else }}
+{{- $name := default .Chart.Name .Values.nameOverride }}
+{{- if contains $name .Release.Name }}
+{{- .Release.Name | trunc 63 | trimSuffix "-" }}
+{{- else }}
+{{- printf "%s-%s" .Release.Name $name | trunc 63 | trimSuffix "-" }}
+{{- end }}
+{{- end }}
+{{- end }}
+
+{{- define "terdut-server.chart" -}}
+{{- printf "%s-%s" .Chart.Name .Chart.Version | replace "+" "_" | trunc 63 | trimSuffix "-" }}
+{{- end }}
+
+{{- define "terdut-server.labels" -}}
+helm.sh/chart: {{ include "terdut-server.chart" . }}
+{{ include "terdut-server.selectorLabels" . }}
+{{- if .Chart.AppVersion }}
+app.kubernetes.io/version: {{ .Chart.AppVersion | quote }}
+{{- end }}
+app.kubernetes.io/managed-by: {{ .Release.Service }}
+{{- end }}
+
+{{- define "terdut-server.selectorLabels" -}}
+app.kubernetes.io/name: {{ include "terdut-server.name" . }}
+app.kubernetes.io/instance: {{ .Release.Name }}
+{{- end }}