Add FaceRecognizer class that loads face encodings from the database,
supports runtime add_encoding(), and matches new encodings by L2 distance.
face_recognition import is deferred so the class works without dlib installed.
FaceResult dataclass carries profile_id, name, confidence, crop, and bbox.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
