feat(docker): rename web service to app, add nginx as web

Reorganize Docker Compose services: the Django/gunicorn container is now
`app` and nginx is `web`, better reflecting their roles. Add a dedicated
gunicorn configuration and install curl in the runtime image for health
checks.

Update documentation to reflect:
- Neo4j migration from ariel.incus to a dedicated umbriel.incus instance
- Rationale for requiring a dedicated Neo4j instance (single-tenancy
  assumptions, label/index isolation, schema ownership)
- New service naming in compose commands and log tailing examples

docker/entrypoint.sh

@@ -10,6 +10,7 @@ case "$1" in
   web)
     # Django REST API + admin (gunicorn → wsgi).
     exec gunicorn \
+      --config /app/docker/gunicorn.conf.py \
       --bind 0.0.0.0:8000 \
       --workers "${GUNICORN_WORKERS:-3}" \
       --access-logfile - \