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/values.yaml

@@ -0,0 +1,52 @@
+backend:
+  image:
+    repository: registry.digitalocean.com/lunarfront/lunarfront-app
+    tag: latest
+    pullPolicy: Always
+  port: 8000
+  resources:
+    requests:
+      cpu: 100m
+      memory: 256Mi
+    limits:
+      cpu: 1000m
+      memory: 512Mi
+
+frontend:
+  image:
+    repository: registry.digitalocean.com/lunarfront/lunarfront-frontend
+    tag: latest
+    pullPolicy: Always
+  port: 80
+  resources:
+    requests:
+      cpu: 50m
+      memory: 64Mi
+    limits:
+      cpu: 200m
+      memory: 128Mi
+
+valkey:
+  image:
+    repository: valkey/valkey
+    tag: "8"
+  port: 6379
+  resources:
+    requests:
+      cpu: 50m
+      memory: 64Mi
+    limits:
+      cpu: 200m
+      memory: 128Mi
+
+ingress:
+  host: ""
+  className: nginx
+  tlsIssuer: letsencrypt
+
+imagePullSecrets:
+  - name: registry-lunarfront
+
+# Secrets are expected to exist in-namespace as 'lunarfront-secrets' with keys:
+#   database-url, jwt-secret, redis-url
+# These are created by the manager during provisioning, not by this chart.