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.xtts-test.yml

@@ -0,0 +1,19 @@
+version: '3.8'
+
+services:
+  app-xtts-test:
+    image: audioenginehub-xtts-test:latest
+    build: .
+    container_name: audio-engine-hub_xtts_test
+    restart: unless-stopped
+    volumes:
+      # Mount local app directory for hot-reloading in dev
+      - ./app:/home/appuser/app
+      # Mount models directory to a top-level directory in the container
+      - ./app/models:/models
+      # Mount asset directory to persist generated audio files
+      - ./asset:/home/appuser/app/asset
+    ports:
+      - "8002:8000"
+    env_file:
+      - .env.xtts-test