fix: avoid blocking audio writes

TTS responses were written synchronously from the async presentation pipeline, blocking concurrent requests. Offload byte writes to a worker thread and add a regression test that keeps the event loop responsive.

surfsense_backend/app/utils/file_io.py

@@ -0,0 +1,7 @@
+import asyncio
+from pathlib import Path
+
+
+async def write_bytes(path: str, content: bytes) -> None:
+    """Write bytes without blocking the event loop."""
+    await asyncio.to_thread(Path(path).write_bytes, content)