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-engine/lib/Zonemaster/Engine/Exception.pm

@@ -0,0 +1,51 @@
+package Zonemaster::Engine::Exception;
+
+use v5.16.0;
+use warnings;
+
+use version; our $VERSION = version->declare("v1.0.3");
+
+use Class::Accessor "antlers";
+
+use overload '""' => \&string;
+
+has 'message' => ( is => 'ro', isa => 'Str', required => 1 );
+
+sub string {
+    my ( $self ) = @_;
+
+    return $self->message;
+}
+
+1;
+
+=head1 NAME
+
+Zonemaster::Engine::Exception -- base class for Zonemaster::Engine exceptions
+
+=head1 SYNOPSIS
+
+   die Zonemaster::Engine::Exception->new({ message => "This is an exception" });
+
+=head1 ATTRIBUTES
+
+=over
+
+=item message
+
+A string attribute holding a message for possible human consumption.
+
+=back
+
+=head1 METHODS
+
+=over
+
+=item string()
+
+Method that stringifies the object by returning the C<message> attribute.
+Stringification is overloaded to this.
+
+=back
+
+=cut