feat: add voice co-creation session skeleton

backend/app/services/voice_session_storage.py

@@ -0,0 +1,48 @@
+"""Voice co-creation session storage helpers."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from app.core.config import settings
+
+
+def session_storage_dir(session_id: str) -> Path:
+    """Return the storage directory for one voice session."""
+
+    return Path(settings.voice_session_storage_dir) / session_id
+
+
+def build_turn_user_audio_path(session_id: str, turn_index: int, suffix: str) -> Path:
+    """Build the persisted path for one user-uploaded turn audio file."""
+
+    normalized_suffix = suffix.lstrip(".") or "webm"
+    return session_storage_dir(session_id) / f"turn-{turn_index:03d}-user.{normalized_suffix}"
+
+
+def build_turn_assistant_audio_path(session_id: str, turn_index: int) -> Path:
+    """Build the persisted path for one generated assistant turn audio file."""
+
+    return session_storage_dir(session_id) / f"turn-{turn_index:03d}-assistant.mp3"
+
+
+def write_session_audio(path: Path, audio_data: bytes) -> str:
+    """Persist session audio bytes atomically and return the saved path."""
+
+    path.parent.mkdir(parents=True, exist_ok=True)
+    temp_path = path.with_suffix(f"{path.suffix}.tmp")
+    temp_path.write_bytes(audio_data)
+    temp_path.replace(path)
+    return str(path)
+
+
+def read_session_audio(audio_path: str) -> bytes:
+    """Read persisted session audio bytes."""
+
+    return Path(audio_path).read_bytes()
+
+
+def session_audio_exists(audio_path: str | None) -> bool:
+    """Whether one stored session audio file currently exists."""
+
+    return bool(audio_path) and Path(audio_path).is_file()