Vite inlines import.meta.env.PUBLIC_* at build time, not runtime.
Without this, the frontend falls back to localhost:8099 which doesn't
exist from the browser. Empty string means same-origin requests,
letting Caddy route /api/* to the backend.
Update frontend Dockerfile prod stage from Caddy static to Node.js
running Astro SSR server. Configure caddy-docker-proxy labels to
route /api/* to backend and everything else to frontend on the same
domain. Support empty PUBLIC_API_URL for same-origin API calls.
