Feature/edgetpu backend

[matteius]

Support for the Coral TPU -- tested on Raspberry Pi 5.

Note: I had to for and update update libedgetpu to get things working on modern tensorflow.

INFO:     127.0.0.1:58172 - "POST /api/v1/detect?backend=edgetpu&confidence_threshold=0.5&return_image=false HTTP/1.1" 200 OK
2025-12-13 22:58:41,725 - api.v1.endpoints.detection - INFO - Detection request: backend=edgetpu, confidence=0.5, filename=lightnvr_api_detection_24958.jpg
2025-12-13 22:58:41,725 - backends.edgetpu.backend - INFO - Attempting to load EdgeTPU as auto
2025-12-13 22:58:41,765 - backends.edgetpu.backend - INFO - EdgeTPU found and loaded successfully
2025-12-13 22:58:41,776 - backends.edgetpu.backend - INFO - EdgeTPU backend initialized: model=ssd_mobilenet_v2_coco_quant_postprocess_edgetpu.tflite, type=ssd, device=auto, input_shape=[  1 300 300   3], tflite_source=tensorflow
2025-12-13 22:58:41,888 - api.v1.endpoints.detection - INFO - Detection completed: 1 objects found in 33.8ms
2025-12-13 22:58:41,888 - api.v1.endpoints.detection - INFO - Detected objects: {'person': 1}
2025-12-13 22:58:41,888 - api.v1.endpoints.detection - INFO - Response prepared: 1 detections, 34.1ms total