Add speaker identification with Resemblyzer

Adds voice-based speaker ID triggered by YAMNet speech detection.
New speaker_id.py module with SQLite-backed voice enrollment and
cosine similarity matching. Endpoints: POST /speakers/enroll,
POST /speakers/enroll-from-mic, GET /speakers, DELETE /speakers/{name}.
Orange LED animation during enrollment.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -5,6 +5,9 @@ Hey-Vivi_*/
 # ML models (downloaded separately)
 models/*.tflite
 
+# Speaker voice database
+voices.db
+
 # Python
 __pycache__/
 *.py[cod]