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

Dockerfile.frontend

@@ -20,6 +20,8 @@ RUN bun run build
 
 FROM nginx:alpine
 COPY --from=build /app/packages/admin/dist /usr/share/nginx/html
-COPY nginx.conf /etc/nginx/conf.d/default.conf
+# nginx docker image processes templates in /etc/nginx/templates/ with envsubst at startup
+COPY nginx.conf /etc/nginx/templates/default.conf.template
+ENV BACKEND_URL=http://localhost:8000
 EXPOSE 80
 CMD ["nginx", "-g", "daemon off;"]