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/SeverityLevelDefinitions.md

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+# Severity Level Definitions
+
+The following severity levels are defined to be assigned to test result messages, one
+at a time, and below it is specified what they intend to mean.
+
+* CRITICAL
+* ERROR
+* WARNING
+* NOTICE
+* INFO
+* DEBUG
+* DEBUG2
+* DEBUG3
+
+
+## CRITICAL
+
+A result at level CRITICAL means that the zone being tested has one or more problems
+that are so severe that it is not possible to even test it. Examples can be that the
+name requested is not syntactically possible, that it's situated under a domain that
+does not exist, or that none of its nameservers are responding.
+    
+A zone with one or more CRITICAL errors can reasonably be said not to exist.
+
+
+## ERROR
+
+A result at level ERROR means a problem that is very likely (or possibly certain) to
+negatively affect the function of the zone being tested, but not so severe that the
+entire zone becomes unresolvable. Examples of ERROR level problems are nameservers
+that do not respond to queries over TCP, only having a single nameserver and using a
+reserved algorithm for DNSKEY records.
+
+
+## WARNING
+
+A result at level WARNING means something that will under some circumstances be a
+problem, but that is unlikely to be noticed by a casual user. Problems at this level
+may be an extra nameserver listed at the parent side, or unsuitable time values in a
+SOA record.
+
+
+## NOTICE
+
+A result at level NOTICE means something that should be known by the zone's
+administrator but that need not necessarily be a problem at all. An example of this
+can be that different name servers for the zone reports slightly different SOA serial
+numbers, which is a perfectly natural consequence of propagation delays but that the
+admin should be aware of.
+
+
+## INFO
+
+A result at level INFO is something that may be of interest to the zone's administrator
+but that definitely does not indicate a problem. Examples of this can be name server
+software versions, the fact that the zone uses DNSCurve encryption or that a name server
+does not recurse.
+
+
+## DEBUG
+
+Results at level DEBUG are related to the zone being tested, but not normally of
+interest to anyone. Implementors of tests are encouraged to be generous with the
+production of these messages. Examples can be that a test section started running, that
+an ordinary action performed as expected or that a value was inside its normal range.
+
+
+## DEBUG2
+
+Results at level DEBUG2 are produced by the testing framework itself, and are intended
+to only be displayed when trying to identify and diagnose unusual and hard to understand
+problems in a zone. This level includes at least one line of information for every DNS
+query sent and response received.
+
+
+## DEBUG3
+
+Results at level DEBUG3 have all the same attributes as those for DEBUG2, and are
+even more voluminous. At this level, full string representations of all received
+DNS packets will be included. At the time of writing, a test run for the same domain
+produces about fifty times as much output on level DEBUG3 as it does on level DEBUG
+and a thousand times as much as on the default NOTICE level.