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

patches/fix-misaki-espeak.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+# Patch misaki/espeak.py to work with newer phonemizer versions
+# The newer phonemizer API doesn't have set_data_path() method
+
+MISAKI_FILE="/opt/venv/lib/python3.11/site-packages/misaki/espeak.py"
+
+if [ -f "$MISAKI_FILE" ]; then
+    echo "Patching misaki/espeak.py for phonemizer compatibility..."
+
+    # Comment out the problematic line
+    sed -i 's/^EspeakWrapper\.set_data_path(espeakng_loader\.get_data_path())/# EspeakWrapper.set_data_path(espeakng_loader.get_data_path())  # Patched: not needed with current phonemizer/' "$MISAKI_FILE"
+
+    echo "Patch applied successfully!"
+else
+    echo "Warning: misaki/espeak.py not found at $MISAKI_FILE"
+fi
+
+# Download SpaCy model for Kokoro text processing
+echo "Downloading SpaCy en_core_web_sm model for Kokoro..."
+python3.11 -m spacy download en_core_web_sm
+echo "SpaCy model downloaded successfully!"
+
+# Create symlink for espeak-ng-data to fix hardcoded path issue
+echo "Creating espeak-ng-data symlink..."
+mkdir -p /home/runner/work/espeakng-loader/espeakng-loader/espeak-ng/_dynamic/share/
+ln -sf /usr/lib/x86_64-linux-gnu/espeak-ng-data /home/runner/work/espeakng-loader/espeakng-loader/espeak-ng/_dynamic/share/espeak-ng-data
+echo "Symlink created successfully!"