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/backend-service.yaml

@@ -0,0 +1,13 @@
+apiVersion: v1
+kind: Service
+metadata:
+  name: {{ .Release.Name }}-backend
+  namespace: {{ .Release.Namespace }}
+  labels:
+    {{- include "lunarfront.labels" . | nindent 4 }}
+spec:
+  selector:
+    app: {{ .Release.Name }}-backend
+  ports:
+    - port: {{ .Values.backend.port }}
+      targetPort: {{ .Values.backend.port }}