Refactor backend integration and service architecture

- Removed the backend client compatibility wrapper and associated methods to streamline backend integration.
- Updated session management to utilize control plane gateways and runtime configuration providers.
- Adjusted TTS service implementations to remove the EdgeTTS service and simplify service dependencies.
- Enhanced documentation to reflect changes in backend integration and service architecture.
- Updated configuration files to remove deprecated TTS provider options and clarify available settings.

engine/app/config.py

@@ -71,7 +71,7 @@ class Settings(BaseSettings):
     # TTS Configuration
     tts_provider: str = Field(
         default="openai_compatible",
-        description="TTS provider (edge, openai_compatible, siliconflow, dashscope)"
+        description="TTS provider (openai_compatible, siliconflow, dashscope)"
     )
     tts_api_url: Optional[str] = Field(default=None, description="TTS provider API URL")
     tts_model: Optional[str] = Field(default=None, description="TTS model name")