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/core/ports/tts.py

@@ -0,0 +1,41 @@
+"""TTS extension port contracts."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import AsyncIterator, Optional, Protocol
+
+from services.base import TTSChunk
+
+
+@dataclass(frozen=True)
+class TTSServiceSpec:
+    """Resolved runtime configuration for TTS service creation."""
+
+    provider: str
+    voice: str
+    sample_rate: int
+    speed: float = 1.0
+    api_key: Optional[str] = None
+    api_url: Optional[str] = None
+    model: Optional[str] = None
+    mode: str = "commit"
+
+
+class TTSPort(Protocol):
+    """Port for speech synthesis providers."""
+
+    async def connect(self) -> None:
+        """Establish connection to TTS provider."""
+
+    async def disconnect(self) -> None:
+        """Release TTS resources."""
+
+    async def synthesize(self, text: str) -> bytes:
+        """Synthesize complete PCM payload for text."""
+
+    async def synthesize_stream(self, text: str) -> AsyncIterator[TTSChunk]:
+        """Stream synthesized PCM chunks for text."""
+
+    async def cancel(self) -> None:
+        """Cancel an in-flight synthesis request."""