docs: video-to-sfx returns audio only (no muxed video) (#5)

The dedicated video-to-audio endpoint no longer returns a muxed video, so
video-to-sfx yields audio only. Fix the README example (which='video' would
now raise) and note that SfxResult.video is retained for compatibility but is
no longer populated.

Author: spencer-zqian

README.md

@@ -90,8 +90,7 @@ task = client.video_to_sfx.submit(
     audio_format="wav",
 )
 result = client.tasks.wait(task.task_id, poll_interval=2.0, timeout=600.0)
-result.save("audio.wav")
-result.save("with_audio.mp4", which="video")  # video-to-sfx also returns the muxed video
+result.save("audio.wav")  # video-to-sfx returns the generated audio only
 ```
 
 `tasks.get(task_id)` fetches state once and never raises on a failed task;

src/sonilo/types.py

@@ -60,6 +60,8 @@ class SfxResult:
     status: str
     type: Optional[str] = None
     audio: Optional[SfxMedia] = None
+    # video-to-sfx now returns audio only; `video` is kept for backward
+    # compatibility but is no longer populated by the API.
     video: Optional[SfxMedia] = None
     cost: Optional[float] = None
     error: Optional[Dict[str, Any]] = None