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

test_run.sh

@@ -1,7 +1,8 @@
 #!/bin/bash
 set -euo pipefail
 
-CMD="PYTHONPATH=/media/jetson/DATA/.Codes/chicken-counting-sukawarna-det/src /media/jetson/DATA/karung-sukawarna/venv/bin/python -m chicken_counter.cli"
+export PYTHONPATH=/media/jetson/DATA/.Codes/chicken-counting-sukawarna-det/src
+PYTHON=/media/jetson/DATA/karung-sukawarna/venv/bin/python
 
 dates=(
     "2026-06-10" "2026-06-11" "2026-06-12" "2026-06-13" "2026-06-14"
@@ -10,5 +11,5 @@ dates=(
 
 for date in "${dates[@]}"; do
     echo "=== Processing $date ==="
-    $CMD batch --config configs/cycle7_batch.yaml --date "$date" --no-video
+    $PYTHON -m chicken_counter.cli batch --config configs/cycle7_batch.yaml --date "$date" --no-video
 done