[add] Load voice parameters from .env file

Configure Piper TTS synthesis via environment variables (speaker_id,
length_scale, noise_scale, noise_w_scale, volume) loaded from .env
with python-dotenv. Includes .env.example as reference template.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -8,6 +8,7 @@ dependencies = [
     "piper-tts>=1.2.0",
     "vosk>=0.3.45",
     "pyaudio>=0.2.14",
+    "python-dotenv>=1.0.0",
 ]
 
 [build-system]