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-ldns/lib/Zonemaster/LDNS/RR/SIG.pm

@@ -0,0 +1,58 @@
+package Zonemaster::LDNS::RR::SIG;
+
+use strict;
+use warnings;
+
+use parent 'Zonemaster::LDNS::RR';
+
+1;
+
+=head1 NAME
+
+Zonemaster::LDNS::RR::SIG - Type SIG record
+
+=head1 DESCRIPTION
+
+A subclass of L<Zonemaster::LDNS::RR>, so it has all the methods of that class available in addition to the ones documented here.
+
+=head1 METHODS
+
+=over
+
+=item typecovered()
+
+Returns a string with the name of the RR type this signature covers.
+
+=item algorithm()
+
+Returns the algorithm number.
+
+=item labels()
+
+Returns the number of labels that was used to calculate the signature.
+
+=item origttl()
+
+Returns the original TTL value.
+
+=item expiration()
+
+Returns the expiration time, as a time_t.
+
+=item inception()
+
+Returns the inception time, as a time_t.
+
+=item keytag()
+
+Returns the keytag.
+
+=item signer()
+
+Returns the signer name.
+
+=item signature()
+
+Returns the cryptographic signture in binary form.
+
+=back