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>

app.py

@@ -6,6 +6,15 @@ Flask application and shared state.
 
 from __future__ import annotations
 
+import os as _os
+if _os.environ.get('INTERCEPT_USE_GEVENT') == '1':
+    try:
+        from gevent import monkey as _monkey
+        _monkey.patch_all(subprocess=False)
+    except ImportError:
+        pass
+del _os
+
 import sys
 import site