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>

README.md

@@ -44,14 +44,16 @@ On first run, visit http://localhost:2001 and the setup wizard will guide you th
 ### First Run
 
 ```bash
-# Create a config file (the setup wizard runs if you skip this)
-cp config.json.example config.json
-
-# Build and start
+# Build and start — no config step needed
 docker compose up -d
 ```
 
-The web UI is available at http://localhost:2001.
+A config file is created automatically in `data/` on first run, then the
+browser **setup wizard** walks you through receiver, location, and password
+setup. The web UI is available at http://localhost:2001.
+
+A `/health` endpoint (used by the container healthcheck) reports status,
+version, and receiver/flight counts: `curl http://localhost:2001/health`.
 
 ### Useful Commands
 
@@ -82,11 +84,11 @@ The following paths are bind-mounted from the repo directory and persist across
 
 | Path | Contents |
 |------|----------|
-| `config.json` | Server configuration and credentials |
+| `data/` | Server configuration and credentials (`config.json`, auto-seeded) |
 | `images/` | Aircraft and UI sprites (including uploads) |
 | `backgrounds/` | Theme background images (including custom themes) |
 
-Host networking (`network_mode: host`) is used so the server can auto-scan your LAN for ADS-B receivers.
+Host networking (`network_mode: host`) is used so the server can auto-scan your LAN for ADS-B receivers. Auto-scan skips oversized subnets (larger than /20, e.g. a Docker bridge `172.17.0.0/16`); if your receiver lives on such a network, set it explicitly via the admin Receivers tab.
 
 ## Requirements