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-backend/docs/internal-documentation/maintenance/Garbage-Collection-Testing.md

@@ -0,0 +1,59 @@
+# Testing instructions for the Garbage Collection feature of the Zonemaster Backend module
+
+## Introduction
+The purpose of this instruction is to serve as a notice for manual testing of the new garbage collection feature at release time.
+
+## Testing the unfinished tests garbage collection feature
+
+
+1. Start a test and wait for it to be finished
+
+     ```
+     SELECT hash_id, progress FROM test_results LIMIT 1;
+     ```
+     Should return:
+
+     ```
+          hash_id      | progress 
+     ------------------+----------
+     3f7a604683efaf93 |      100
+     (1 row)
+     ```
+
+2. Simulate a crashed test
+     ```
+     UPDATE test_results SET progress = 50, test_start_time = '2020-01-01' WHERE hash_id = '3f7a604683efaf93';
+     ```
+
+3. Check that the backend finishes the test with a result stating it was unfinished
+
+     ```
+     SELECT hash_id, progress FROM test_results WHERE hash_id = '3f7a604683efaf93';
+     ```
+     Should return a finished result:
+     ```
+          hash_id      | progress 
+     ------------------+----------
+     3f7a604683efaf93 |      100
+     (1 row)
+     ```
+
+4. Ensure the test result contains the backend generated critical message:
+     ```
+     {"tag":"UNABLE_TO_FINISH_TEST","level":"CRITICAL","timestamp":"300","module":"BACKEND_TEST_AGENT"}
+     ```
+
+     ```
+     SELECT hash_id, progress FROM test_results WHERE hash_id = '3f7a604683efaf93' AND results::text like '%UNABLE_TO_FINISH_TEST%';
+     ```
+     _Remark: for MySQL queries remove the `::text` from all queries_
+
+     Should return:
+     ```
+          hash_id      | progress 
+     ------------------+----------
+     3f7a604683efaf93 |      100
+     (1 row)
+
+     ```
+