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

Dockerfile

@@ -62,6 +62,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
         zlib1g \
         libssl3 \
         ca-certificates \
+        curl \
     && rm -rf /var/lib/apt/lists/*
 
 ENV PYTHONDONTWRITEBYTECODE=1 \
@@ -77,6 +78,7 @@ COPY --from=builder /usr/local/bin /usr/local/bin
 WORKDIR /app
 COPY --from=builder /build/mnemosyne /app
 COPY docker/entrypoint.sh /usr/local/bin/entrypoint.sh
+COPY docker/gunicorn.conf.py /app/docker/gunicorn.conf.py
 RUN chmod +x /usr/local/bin/entrypoint.sh
 
 # Non-root user for everything that runs in this image. uid:gid 1000:1000