feat: Add XTTS v2 support, refactor Docker/GPU infra, and improve Piper engine

- Add XTTS v2 configuration to .env.example
- Refactor Dockerfile to multi-stage build with CUDA 12.1 support
- Update Makefile with Kokoro and XTTS test environment targets
- Refactor Piper engine (app/engines/piper.py) to use python module execution
- Add comprehensive documentation for Kokoro and XTTS plans
- Add helper scripts and patches for build process

docker-compose.yml

@@ -12,7 +12,8 @@ services:
       - ./app/models:/models
       # Mount asset directory to persist generated audio files
       - ./asset:/home/appuser/app/asset
-    command: ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000", "--reload"]
+    # Use Dockerfile's CMD (gunicorn) instead of uvicorn for production
+    # command: ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]
     ports:
       - "${APP_PORT:-8000}:8000"
     env_file: