feat(nginx): bake default white-label routing config, closes #1 (#2)

## Summary

- Add `nginx.conf.template` with full routing: admin/CMS surfaces → `cms:5555`, uploads → `cms:5555` (cached), `/api` → `shop:9999` (no cache), `/` → `shop:9999` (cached)
- Update `docker-entrypoint.sh` to render template via `envsubst` at startup; skip if adapter has already mounted its own config
- Update `Dockerfile` to `COPY nginx.conf.template` into image
- Bump CHANGELOG to v0.1.1

## Test plan

- [ ] Build image locally: `docker build -t libreshop/nginx:test libreshop/nginx/`
- [ ] Smoke test white-label default: no env overrides → confirm rendered `/etc/nginx/nginx.conf` contains `cms:5555` and `shop:9999`
- [ ] Smoke test env override: `NGINX_UPSTREAM_SHOP=shop:8080` → confirm substitution
- [ ] Smoke test adapter override: mount custom `nginx.conf` via compose `configs:` → confirm "adapter-provided nginx.conf detected" log line
- [ ] Deploy libreshop/demo with `nginx:v0.1.1` → confirm `/admin` returns 200

Closes #1

Co-authored-by: Michael Czechowski <michael.c@re-cinq.com>
Reviewed-on: #2

CHANGELOG.md

@@ -2,7 +2,16 @@
 
 All notable changes to libreshop/nginx are documented here.
 
-## Unreleased
+## v0.1.1 (Unreleased)
 
+### Added
+- Default white-label `nginx.conf` baked into the image. Routes the standard libreshop surface out of the box: `/admin`, `/upload`, `/content-manager`, `/content-type-builder`, `/i18n`, `/email`, `/user-permissions`, `/users-permissions`, `/documentation`, `/plugins`, `/uploads` → `${NGINX_UPSTREAM_CMS:-cms:5555}`; `/api` and `/` → `${NGINX_UPSTREAM_SHOP:-shop:9999}`. Cache zones, X-Forwarded headers, and a `/health` endpoint included.
+- `docker-entrypoint.sh` envsubst pass over `nginx.conf.template` at startup with `NGINX_UPSTREAM_SHOP`, `NGINX_UPSTREAM_CMS`, `NGINX_CACHE_LIFETIME` (default `1h`).
+- Adapter-override detection: if `/etc/nginx/nginx.conf` was replaced via compose `configs:` (i.e. doesn't carry the stock nginx `worker_processes  auto;` line), the entrypoint leaves it untouched.
+
+### Why
+`libreshop/demo` standalone preview returned 404 on `/admin` because the image had no default routing. Adapters like mp work fine because they mount their own `nginx.conf`, but the toolkit must be runnable without an adapter.
+
+## Unreleased
 - Extracted from `mp/nginx/` (2026-04-29). The component history before
   the extraction lives in the `muellerprints` repository.