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/docs/public/using/backend/README.md

@@ -0,0 +1,10 @@
+# Using the Backend
+
+* [Using Zonemaster-Backend JSON-RPC API](Using-Zonemaster-Backend-JSON-RPC-API.md)
+* [Using Zonemaster-Backend for batch testing](Using-Zonemaster-Backend-for-batch-testing.md)
+* [RPCAPI reference]
+* [Collecting metrics][Telemetry]
+* [Using Zonemaster-Backend Docker container](Using-Zonemaster-Backend-Docker.md)
+
+[RPCAPI reference]: rpcapi-reference.md
+[Telemetry]:        telemetry.md