Fix dashboard freeze: waitress, reduce frame refresh, atomic reads

- Switch Flask dev server -> waitress (8 threads, production WSGI)
- Only refresh frame.jpg when frame_index changes, not every second
- Safety try/except on send_file for edge-case file conflicts
- Fix test_run.sh: export PYTHONPATH, use venv python
- Update service file to use venv python

chicken-dashboard.service

@@ -6,7 +6,7 @@ After=network.target
 Type=simple
 User=dsutanto
 WorkingDirectory=/media/jetson/DATA/.Codes/chicken-counting-sukawarna-det
-ExecStart=/usr/bin/python3 dashboard.py --port 8080 --db chicken_counts.db
+ExecStart=/media/jetson/DATA/karung-sukawarna/venv/bin/python dashboard.py --port 8080 --db chicken_counts.db
 Restart=always
 RestartSec=5
 Environment=PYTHONUNBUFFERED=1