feat: multi-model modes (A/B/C/D, default B) + box counting + manual-mode banner

.env.example

@@ -17,3 +17,8 @@ FLASK_DEBUG=false
 
 # Stream URL
 RTSP_URL=rtsp://user:pass@192.168.192.209:8554/camera_stream_640
+
+# Model pipeline mode: A=combined only; B=v4 truck + yolo11n sack+box (default);
+# C=A + yolo11n box-only; D=v4 truck + best sack-only + yolo11n box-only.
+# Dashboard switches persist here and apply on next service restart.
+MODEL_MODE=B