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>

docker-entrypoint.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+# Seed a config file on first run so `docker compose up` works on a fresh
+# clone with no manual steps. The config lives on a mounted data volume so it
+# persists across container rebuilds.
+set -e
+
+: "${ADSBIT_CONFIG:=/app/config.json}"
+CONFIG_DIR=$(dirname "$ADSBIT_CONFIG")
+mkdir -p "$CONFIG_DIR"
+
+if [ ! -f "$ADSBIT_CONFIG" ]; then
+    echo "[entrypoint] No config at $ADSBIT_CONFIG — seeding from config.json.example"
+    cp /app/config.json.example "$ADSBIT_CONFIG"
+fi
+
+exec python3 server.py