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/configuration/backend-environment-variables.md

@@ -0,0 +1,35 @@
+# Environment variables
+
+## Variable used by both RPCAPI daemon and Test Agent daemon
+
+* `ZONEMASTER_BACKEND_CONFIG_FILE`: Set a custom path for the backend
+  configuration file.
+
+## Variables used by RPCAPI daemon only
+
+* `ZM_BACKEND_RPCAPI_LOGLEVEL`: Configure the log level, `trace` by default.
+  Accepted values are:
+  * `trace`
+  * `debug`
+  * `info` (also accepted: `inform`)
+  * `notice`
+  * `warning` (also accepted: `warn`)
+  * `error` (also accepted: `err`)
+  * `critical` (also accepted: `crit`, `fatal`)
+  * `alert`
+  * `emergency`
+* `ZM_BACKEND_RPCAPI_LOGJSON`: Setting it to any thruthy value (non-empty
+  string or non-zero number) will configure the logger to log in JSON format,
+  undefined by default.
+
+## Variables used by unit test scripts
+
+* `TARGET`: Set the database to use. If empty or undefined, "SQLite" will be
+  assumed. Accepted values are:
+  * `SQLite`
+  * `PostgreSQL`
+  * `MySQL`
+
+* `ZONEMASTER_RECORD`: Setting it to any thruthy value (non-empty string or
+  non-zero number) will record the data from the test to a file. Otherwise the
+  data is loaded from a file.