feat: add full Zonemaster stack with Docker and Spanish UI

- Clone all 5 Zonemaster component repos (LDNS, Engine, CLI, Backend, GUI)
- Dockerfile.backend: 8-stage multi-stage build LDNS→Engine→CLI→Backend
- Dockerfile.gui: Astro static build served via nginx
- docker-compose.yml: backend (internal) + frontend (port 5353)
- nginx.conf: root redirects to /es/, /api/ proxied to backend
- zonemaster-gui/config.ts: defaultLanguage set to 'es' (Spanish)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster-backend/CONTRIBUTING.md

@@ -0,0 +1,30 @@
+# Contributing to Zonemaster::Backend
+
+Contribution to this repository is welcome. Contribution can be either an issue
+report or a code or a documentation update. Also see the information in the
+[main README][Zonemaster/Zonemaster README] in the main Zonemaster respository.
+
+## Issue
+
+First search for a similar issue in the [issues list]. If a relevant issue is
+found, add your information as a comment. If no relevant issue is found, create
+[a new issue][create issue]. Give as many details as you have and describe, if
+possible, how the issue can be reproduced.
+
+## Pull request
+
+If you would like to contribute an update, first please look for issues and open
+[pull requests] that are about the same thing. If nothing relevant is found or
+you have a different solution, create [a new pull request][create pull request].
+Creating a pull request assumes that you have your proposal in a fork repository.
+
+When you create a pull request, please always start with the `develop` branch
+and create the pull request against the same branch.
+
+
+[issues list]:                        https://github.com/zonemaster/zonemaster-backend/issues
+[create issue]:                       https://github.com/zonemaster/zonemaster-backend/issues/new
+[pull requests]:                      https://github.com/zonemaster/zonemaster-backend/pulls
+[create pull request]:                https://github.com/zonemaster/zonemaster-backend/compare
+[Zonemaster/Zonemaster README]:       https://github.com/zonemaster/zonemaster#readme
+