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-deployment.yaml

@@ -0,0 +1,59 @@
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: {{ .Release.Name }}-backend
+  namespace: {{ .Release.Namespace }}
+  labels:
+    {{- include "lunarfront.labels" . | nindent 4 }}
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: {{ .Release.Name }}-backend
+  template:
+    metadata:
+      labels:
+        app: {{ .Release.Name }}-backend
+    spec:
+      imagePullSecrets:
+        {{- toYaml .Values.imagePullSecrets | nindent 8 }}
+      containers:
+        - name: backend
+          image: "{{ .Values.backend.image.repository }}:{{ .Values.backend.image.tag }}"
+          imagePullPolicy: {{ .Values.backend.image.pullPolicy }}
+          ports:
+            - containerPort: {{ .Values.backend.port }}
+          env:
+            - name: PORT
+              value: {{ .Values.backend.port | quote }}
+            - name: NODE_ENV
+              value: production
+            - name: DATABASE_URL
+              valueFrom:
+                secretKeyRef:
+                  name: lunarfront-secrets
+                  key: database-url
+            - name: REDIS_URL
+              valueFrom:
+                secretKeyRef:
+                  name: lunarfront-secrets
+                  key: redis-url
+            - name: JWT_SECRET
+              valueFrom:
+                secretKeyRef:
+                  name: lunarfront-secrets
+                  key: jwt-secret
+          livenessProbe:
+            httpGet:
+              path: /v1/health
+              port: {{ .Values.backend.port }}
+            initialDelaySeconds: 15
+            periodSeconds: 30
+          readinessProbe:
+            httpGet:
+              path: /v1/health
+              port: {{ .Values.backend.port }}
+            initialDelaySeconds: 5
+            periodSeconds: 10
+          resources:
+            {{- toYaml .Values.backend.resources | nindent 12 }}