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/main.py

@@ -76,7 +76,7 @@ app.add_middleware(
 
 # Active sessions storage
 active_sessions: Dict[str, Session] = {}
-backend_gateway = build_backend_adapter_from_settings()
+control_plane_gateway = build_backend_adapter_from_settings()
 
 # Configure logging
 logger.remove()
@@ -187,7 +187,7 @@ async def websocket_endpoint(websocket: WebSocket):
     session = Session(
         session_id,
         transport,
-        backend_gateway=backend_gateway,
+        control_plane_gateway=control_plane_gateway,
         assistant_id=assistant_id,
     )
     active_sessions[session_id] = session
@@ -272,7 +272,7 @@ async def webrtc_endpoint(websocket: WebSocket):
     session = Session(
         session_id,
         transport,
-        backend_gateway=backend_gateway,
+        control_plane_gateway=control_plane_gateway,
         assistant_id=assistant_id,
     )
     active_sessions[session_id] = session