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.example

@@ -1,6 +1,6 @@
 # Comma-separated list of engines to activate.
-# Available options (potentially): piper, styletts, f5_tts, chattts
-ACTIVE_ENGINES='["piper", "styletts"]'
+# Available options (potentially): piper, styletts, f5_tts, chattts, kokoro, xtts
+ACTIVE_ENGINES='["piper", "styletts", "xtts"]'
 
 # Server configuration
 HOST=0.0.0.0
@@ -9,3 +9,12 @@ PORT=8000
 # Piper Engine Configuration
 PIPER_TIMEOUT_SECONDS=30
 FFMPEG_TIMEOUT_SECONDS=60
+
+# Kokoro Engine Configuration
+KOKORO_DEVICE=cuda  # cuda or cpu
+KOKORO_TIMEOUT_SECONDS=30
+
+# XTTS Engine Configuration
+XTTS_ACCEPT_LICENSE=false # Set to true to accept the Coqui XTTS license
+XTTS_DEVICE=cpu # cuda or cpu
+VOICES_DIR=app/asset/voices # Directory where reference audio files for voice cloning are stored