fix(compose): make web/data writable for deploy.json updates

The web docroot is mounted read-only; mount web/data read-write so the
release workflow's deploy.json refresh can write into the nginx container.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -57,6 +57,9 @@ services:
     restart: unless-stopped
     volumes:
       - ./web:/usr/share/nginx/html:ro
+      # data/ stays writable so the release workflow can refresh deploy.json
+      # (the rest of the docroot is read-only)
+      - ./web/data:/usr/share/nginx/html/data
       - ./web/nginx.conf:/etc/nginx/conf.d/default.conf:ro
     ports:
       - "3200:80"     # Web (xetup.x9.cz via reverse proxy)