feat: add gunicorn + gevent production server via start.sh

Add start.sh as the recommended production entry point with:
- gunicorn + gevent worker for concurrent SSE/WebSocket handling
- CLI flags for port, host, debug, HTTPS, and dependency checks
- Auto-fallback to Flask dev server if gunicorn not installed
- Conditional gevent monkey-patch in app.py via INTERCEPT_USE_GEVENT env var
- Docker CMD updated to use start.sh
- Updated all docs, setup.sh, and requirements.txt accordingly

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/USAGE.md

@@ -172,7 +172,7 @@ Set the following environment variables (Docker recommended):
 ```bash
 INTERCEPT_ADSB_AUTO_START=true \
 INTERCEPT_SHARED_OBSERVER_LOCATION=false \
-sudo -E venv/bin/python intercept.py
+sudo ./start.sh
 ```
 
 **Docker example (.env)**
@@ -518,10 +518,26 @@ INTERCEPT can be configured via environment variables:
 | `INTERCEPT_LOG_LEVEL` | `WARNING` | Log level (DEBUG, INFO, WARNING, ERROR) |
 | `INTERCEPT_DEFAULT_GAIN` | `40` | Default RTL-SDR gain |
 
-Example: `INTERCEPT_PORT=8080 sudo -E venv/bin/python intercept.py`
+Example: `INTERCEPT_PORT=8080 sudo ./start.sh`
 
 ## Command-line Options
 
+### Production server (recommended)
+
+```
+./start.sh --help
+
+  -p, --port PORT    Port to listen on (default: 5050)
+  -H, --host HOST    Host to bind to (default: 0.0.0.0)
+  -d, --debug        Run in debug mode (Flask dev server)
+  --https            Enable HTTPS with self-signed certificate
+  --check-deps       Check dependencies and exit
+```
+
+`start.sh` auto-detects gunicorn + gevent and runs a production WSGI server with cooperative greenlets — this handles multiple SSE streams and WebSocket connections concurrently without blocking. Falls back to the Flask dev server if gunicorn is not installed.
+
+### Development server
+
 ```
 python3 intercept.py --help