nginx: listen on PORT, default 8080; server_tokens off (closes #26)

nginx.conf is now a template the nginx image renders into conf.d at
container start. bin/entrypoint.sh gives nginx PORT, 8080 when unset or
empty, and limits the rendering to PORT with NGINX_ENVSUBST_FILTER, so
$uri, $host and every other nginx variable pass through unchanged. A
PORT nginx cannot listen on stops the container non-zero.
server_tokens off drops the version from the Server header and error
pages. script/frontend-viewport-test renders the template the same way.
EXPOSE still documents 8080; the backend stays on 127.0.0.1:8081.

Model: opus-5-5

TODO.md

@@ -23,6 +23,11 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: nginx listens on `PORT` (issue #26), 8080 when unset: the nginx
+  image renders `nginx.conf` as a template at container start, filling in `PORT`
+  and no other variable. `server_tokens off` keeps the nginx version out of
+  responses. `script/frontend-viewport-test` renders the template the same way.
+  Gzip and a `50x.html` error page are not added
 - 2026-09-28: one container image (issue #52): the root `Dockerfile` builds the
   only image, and `Dockerfile.backend` is gone. nginx serves the frontend on
   port 8080 and proxies `/api/` and `/.well-known/healthcheck` to the backend,