v1.1.1: harden deployment for easier fielding

Make a fresh deploy (especially Docker) work with no manual steps and behave
well on real-world hosts:

- Auto-scan skips subnets larger than /20 (e.g. a docker bridge 172.17.0.0/16
  = 65k hosts) so AUTO discovery no longer stalls startup or hammers the
  network on any host with Docker. Manual scan shares the same MIN_SCAN_PREFIX.
- Docker: config.json is auto-seeded from config.json.example by a new
  docker-entrypoint.sh and persisted on a ./data directory mount — no more
  single-file bind-mount footgun, no manual cp before first run.
- Config path is overridable via ADSBIT_CONFIG (defaults to ./config.json for
  bare metal; /app/data/config.json in the container).
- New /health endpoint (status/version/receivers/flights) + Docker HEALTHCHECK.
- Clean SIGTERM/SIGINT shutdown (docker stop / systemctl stop) — no traceback.
- .gitignore/.dockerignore exclude the data/ config volume.
- README/CLAUDE.md updated; VERSION bumped to 1.1.1.

Verified: image builds, fresh `docker run` seeds config and reports healthy,
oversized subnets skipped in-container, /health responds, graceful shutdown.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -13,9 +13,16 @@ RUN pip install --no-cache-dir -r requirements.txt && \
     rm -rf /var/lib/apt/lists/*
 
 COPY . .
+RUN chmod +x /app/docker-entrypoint.sh
 
 EXPOSE 2001
 
-ENV PYTHONUNBUFFERED=1
+# Persist config on a mounted volume by default (see docker-compose.yml).
+ENV PYTHONUNBUFFERED=1 \
+    ADSBIT_CONFIG=/app/data/config.json
 
-CMD ["python3", "server.py"]
+# Liveness probe against the unauthenticated /health endpoint.
+HEALTHCHECK --interval=30s --timeout=5s --start-period=20s --retries=3 \
+    CMD python3 -c "import urllib.request,sys; sys.exit(0 if urllib.request.urlopen('http://127.0.0.1:2001/health',timeout=4).status==200 else 1)" || exit 1
+
+ENTRYPOINT ["/app/docker-entrypoint.sh"]