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/index.html

@@ -331,7 +331,7 @@
                         <pre><code>git clone https://github.com/smittix/intercept.git
 cd intercept
 ./setup.sh
-sudo -E venv/bin/python intercept.py</code></pre>
+sudo ./start.sh</code></pre>
                     </div>
                     <p class="install-note">Requires Python 3.9+ and RTL-SDR drivers</p>
                 </div>