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/specifications/tests/Zone-TP/zone04.md

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+## ZONE04: SOA 'retry' at least 1 hour
+
+### Test case identifier
+**ZONE04** SOA 'retry' at least 1 hour
+
+### Objective
+
+The SOA retry value is the number of seconds that describes
+minimum time elapsed since a failed zone refresh from the primary
+name server. The SOA refresh value is described
+in section 3.3.13 in [RFC 1035](https://datatracker.ietf.org/doc/html/rfc1035),
+and clarified in section 2.2 of
+[RFC 1912](https://datatracker.ietf.org/doc/html/rfc1912).
+
+Setting the retry value low will increase the DNS traffic between
+the servers, and also increase the load on the master name server.
+
+The [RIPE-203](https://www.ripe.net/publications/docs/ripe-203) recommendation
+for the retry value is 2 hours (7200 seconds). Older DNSCheck code
+had a one hour minimum value (3600 seconds), and this is the minimum
+value we recommend.
+
+### Inputs
+
+The domain name to be tested.
+
+### Ordered description of steps to be taken to execute the test case
+
+1. Retrieve the SOA record from a delegated name server for the domain.
+2. If the answer from step 1 is not authoritative, iterate step 1 until there is an authoritative answer.
+3. Retrieve the retry value from the SOA record.
+4. If the retry value is less than 3600 seconds, this test case fails.
+
+### Outcome(s)
+
+If the retry value is less than 3600 seconds, this test case fails.
+
+### Special procedural requirements
+
+None.
+
+### Intercase dependencies
+
+None.