feat: persist story generation states and cache audio

backend/app/services/audio_storage.py

@@ -0,0 +1,38 @@
+"""Story audio cache storage helpers."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from app.core.config import settings
+
+
+def build_story_audio_path(story_id: int) -> str:
+    """Build the cache path for a story audio file."""
+
+    return str(Path(settings.story_audio_cache_dir) / f"story-{story_id}.mp3")
+
+
+def audio_cache_exists(audio_path: str | None) -> bool:
+    """Whether the cached audio file exists on disk."""
+
+    return bool(audio_path) and Path(audio_path).is_file()
+
+
+def read_audio_cache(audio_path: str) -> bytes:
+    """Read cached story audio bytes."""
+
+    return Path(audio_path).read_bytes()
+
+
+def write_story_audio_cache(story_id: int, audio_data: bytes) -> str:
+    """Persist story audio and return the saved file path."""
+
+    final_path = Path(build_story_audio_path(story_id))
+    final_path.parent.mkdir(parents=True, exist_ok=True)
+
+    temp_path = final_path.with_suffix(".tmp")
+    temp_path.write_bytes(audio_data)
+    temp_path.replace(final_path)
+
+    return str(final_path)