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 sets PORT to 8080 when unset or
empty, and stops with an error before starting anything when PORT is
not digits only: nginx would take a value such as localhost or
unix:/tmp/x.sock as an address and start anyway. NGINX_ENVSUBST_FILTER
limits the rendering to PORT, so $uri, $host and every other nginx
variable pass through unchanged. 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 +23,12 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: nginx listens on `PORT` (issue #26), 8080 when unset or empty: the
+  nginx image renders `nginx.conf` as a template at container start, filling in
+  `PORT` and no other variable. `bin/entrypoint.sh` refuses to start when `PORT`
+  is not digits only. `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
 - 2026-09-29: bounded the report endpoint (issue #20): `POST /api/v1/reports`
   still needs no credentials, but each client address, as resolved through
   `TRUSTED_PROXIES`, may send `REPORTS_PER_MINUTE` (default 60) reports a