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>

docker-compose.yml

@@ -1,6 +1,8 @@
 # INTERCEPT - Signal Intelligence Platform
 # Docker Compose configuration for easy deployment
 #
+# Uses gunicorn + gevent production server via start.sh (handles concurrent SSE/WebSocket)
+#
 # Basic usage (build locally):
 #   docker compose --profile basic up -d --build
 #