Make face detection_threshold effective below 0.5

FaceDetectorYN was created with a hardcoded score_threshold of 0.5, so
configured face_recognition.detection_threshold values below 0.5 had no
effect: lower-confidence faces were discarded inside OpenCV before the
config threshold was applied as a post-filter.

On cameras with low-resolution detect streams (e.g. 736x416 substreams),
real faces routinely score 0.3-0.5 in YuNet, so face recognition never
ran on them even though the embedding model handles such crops well.

Cap the internal threshold at the configured value instead.

frigate/data_processing/real_time/face.py

@@ -127,7 +127,7 @@ class FaceRealTimeProcessor(RealTimeProcessorApi):
             os.path.join(MODEL_CACHE_DIR, "facedet/facedet.onnx"),
             config="",
             input_size=(320, 320),
-            score_threshold=0.5,
+            score_threshold=min(0.5, self.face_config.detection_threshold),
             nms_threshold=0.3,
         )
         self.faces_per_second.start()