add face feature extraction pipeline

models/README.md

@@ -0,0 +1,18 @@
+# Face Embedding Model
+
+`face_recognition_sface_2021dec.onnx` is the OpenCV Zoo SFace model used by
+the default application. It accepts an aligned 112 x 112 BGR face and returns
+a 128-dimensional feature vector. The application L2-normalizes that vector
+before publishing it.
+
+- Source: https://github.com/opencv/opencv_zoo/tree/main/models/face_recognition_sface
+- Download mirror: https://huggingface.co/opencv/face_recognition_sface
+- Upstream project: https://github.com/opencv/opencv
+- Model filename: `face_recognition_sface_2021dec.onnx`
+- SHA-256: `0ba9fbfa01b5270c96627c4ef784da859931e02f04419c829e83484087c34e79`
+- License: Apache License 2.0; see `LICENSE` in this directory.
+
+Keep the license and model metadata with the model when redistributing the
+repository. CMake verifies the bundled model checksum, and the application
+validates its output contract during startup rather than running without feature
+extraction.