feat: extract nginx from mp/nginx — initial libreshop/nginx

Source moved verbatim from mp/nginx/ on 2026-04-29; mp was the first
concrete adapter consuming the libreshop toolkit. Builds and publishes
git.librete.ch/libreshop/nginx on every main / v* push via the standard
.gitea/workflows/build.yml shared across libreshop components.

CHANGELOG.md

@@ -0,0 +1,8 @@
+# Changelog
+
+All notable changes to libreshop/nginx are documented here.
+
+## Unreleased
+
+- Extracted from `mp/nginx/` (2026-04-29). The component history before
+  the extraction lives in the `muellerprints` repository.