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

.env.xtts-test

@@ -0,0 +1,13 @@
+LOG_LEVEL=DEBUG
+
+ACTIVE_ENGINES='["piper", "kokoro", "xtts"]'
+XTTS_ACCEPT_LICENSE=true
+
+HOST=0.0.0.0
+PORT=8000
+
+# Kokoro Engine Configuration
+KOKORO_DEVICE=cuda
+KOKORO_TIMEOUT_SECONDS=30
+
+VOICES_DIR=/home/appuser/app/asset/voices