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/valkey-deployment.yaml

@@ -0,0 +1,24 @@
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: {{ .Release.Name }}-valkey
+  namespace: {{ .Release.Namespace }}
+  labels:
+    {{- include "lunarfront.labels" . | nindent 4 }}
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: {{ .Release.Name }}-valkey
+  template:
+    metadata:
+      labels:
+        app: {{ .Release.Name }}-valkey
+    spec:
+      containers:
+        - name: valkey
+          image: "{{ .Values.valkey.image.repository }}:{{ .Values.valkey.image.tag }}"
+          ports:
+            - containerPort: {{ .Values.valkey.port }}
+          resources:
+            {{- toYaml .Values.valkey.resources | nindent 12 }}