feat: add week 3 audio and timeline enhancements

backend/app/schemas/story_schemas.py

@@ -154,6 +154,16 @@ class StoryImageResponse(StoryStatusMixin):
     image_url: str | None
 
 
+class StoryAudioStatusResponse(StoryStatusMixin):
+    """Audio cache status for one story."""
+
+    story_id: int
+    audio_ready: bool
+    cache_exists: bool
+    cache_size_bytes: int | None = None
+    cache_updated_at: datetime | None = None
+
+
 class StoryAssetRetryRequest(BaseModel):
     """Retry selected generated assets for a story."""