feat: Add OpenAI-compatible TTS endpoint and engines

- Implements POST /v1/audio/speech endpoint (OpenAI API compatible).
- Integrates Kokoro and XTTS engines (including dependencies and implementations).
- Updates main application to register new engines and router.
- Adds unit tests for OpenAI compatibility.
- Updates requirements.txt for new engines.

requirements.txt

@@ -10,11 +10,29 @@ pydantic-settings
 ffmpeg-python
 piper-tts
 
+# Kokoro TTS Engine
+kokoro>=0.9.2
+soundfile
+phonemizer
+scipy
+munch
+# Pin compatible espeakng-loader version for misaki (kokoro dependency)
+espeakng-loader>=0.2.3,<0.2.5
+
+# Coqui XTTS Engine
+TTS
+# Pin transformers to version compatible with TTS library
+transformers<4.42.0
 
 f5-tts
-torch
+# Pin torch to <2.6 to avoid weights_only loading issues with TTS library
+torch<2.6
 torchaudio
 
+# Numba/Numpy compatibility for XTTS/Torch
+numba<0.58
+numpy<1.25
+
 # Development & Testing
 pytest-cov
 pytest-asyncio