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/docs/backend_integration.md

@@ -27,9 +27,8 @@ Assistant config source behavior:
 
 ## Architecture
 
-- Ports: `core/ports/backend.py`
+- Ports: `core/ports/control_plane.py`
 - Adapters: `app/backend_adapters.py`
-- Compatibility wrappers: `app/backend_client.py`
 
 `Session` and `DuplexPipeline` receive backend capabilities via injected adapter
 methods instead of hard-coding backend client imports.