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/ingress.yaml

@@ -0,0 +1,29 @@
+{{- if .Values.ingress.host }}
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: {{ .Release.Name }}
+  namespace: {{ .Release.Namespace }}
+  labels:
+    {{- include "lunarfront.labels" . | nindent 4 }}
+  annotations:
+    kubernetes.io/ingress.class: {{ .Values.ingress.className }}
+    cert-manager.io/cluster-issuer: {{ .Values.ingress.tlsIssuer }}
+spec:
+  ingressClassName: {{ .Values.ingress.className }}
+  tls:
+    - hosts:
+        - {{ .Values.ingress.host }}
+      secretName: {{ .Release.Name }}-tls
+  rules:
+    - host: {{ .Values.ingress.host }}
+      http:
+        paths:
+          - path: /
+            pathType: Prefix
+            backend:
+              service:
+                name: {{ .Release.Name }}-frontend
+                port:
+                  number: 80
+{{- end }}