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/tests/unit/agents/video_presentation/test_file_io.py

@@ -0,0 +1,29 @@
+import asyncio
+import threading
+
+import pytest
+
+from app.utils.file_io import write_bytes
+
+pytestmark = pytest.mark.unit
+
+
+@pytest.mark.asyncio
+async def test_write_bytes_does_not_block_event_loop(monkeypatch, tmp_path):
+    write_started = threading.Event()
+    release_write = threading.Event()
+
+    def blocking_write(_path, _content):
+        write_started.set()
+        release_write.wait(timeout=1)
+        return 5
+
+    monkeypatch.setattr("pathlib.Path.write_bytes", blocking_write)
+
+    task = asyncio.create_task(write_bytes(str(tmp_path / "audio.mp3"), b"audio"))
+    while not write_started.is_set():
+        await asyncio.sleep(0)
+
+    assert not task.done()
+    release_write.set()
+    await task