feature: allow a configurable backend address

[mj0nez]

This PR migrates the Nginx config to a template which allows now the configuration of the backend address, which is necessary if one want changes the compose service or uses a different deployment method / networking stack.

[codebude]

Thanks for your PR. The idea is right: the hardcoded backend:8000 locks the image to one Compose network and service name, which rules out Kubernetes, Portainer and similar setups. Four things before merge, all small:

1. BACKEND_ADDR needs a default. Unset, envsubst leaves the placeholder in and nginx won't start: nginx: [emerg] unknown "backend_addr" variable. Our compose files are updated so the documented path is fine, but anyone pulling the ghcr image directly (k8s, Portainer, own compose, Helm) breaks at container start. A one-liner in frontend/Dockerfile fixes it without losing the configurability:

   ENV BACKEND_ADDR=http://backend:8000

   Tested both ways: unset renders the default and nginx -t passes, set still overrides it.

2. Worth a "no trailing slash" note in the docs. proxy_pass treats the slash as a URI part and strips the location prefix, so http://backend:8000/ sends /api/books upstream as /books. nginx starts fine, which makes it look like a backend bug.

3. Compose: ${BACKEND_ADDR:-http://backend:8000} instead of a literal, matching the ${VAR:-default} pattern already used in these files. Then the upstream can be set from .env instead of editing the compose file, which is closer to the point of the PR.

4. docs/releases.md needs an small entry/update

Happy to re-review when these are in.