feat: add Helm chart and switch image builds to DOCR

- Add chart/ with backend, frontend, valkey deployments, services, and ingress
- Update nginx.conf to use BACKEND_URL env var via envsubst
- Update Dockerfile.frontend to use nginx template mechanism
- Build.yml: switch Docker registry from Gitea to DOCR, add helm package+push step

chart/templates/_helpers.tpl

@@ -0,0 +1,8 @@
+{{- define "lunarfront.name" -}}
+{{- .Release.Name }}
+{{- end }}
+
+{{- define "lunarfront.labels" -}}
+app.kubernetes.io/instance: {{ .Release.Name }}
+app.kubernetes.io/managed-by: {{ .Release.Service }}
+{{- end }}