# Get Audio Chapters

The format-abstraction chapter list for audiobooks, sourced from
`media_items.audio_metadata`. Embedded m4b chapters, folder-of-tracks
books and zip-of-audio books all write the same chapter shape; the
locator `{chapter, audio_offset_seconds}` and this endpoint never learn
which file model produced them.

**Endpoint**: `GET /api/media-items/{id}/chapters`
**Auth**: Required (JWT)

## Response

```json
{
  "media_item_id": "…",
  "chapters": [
    { "index": 0, "title": "Chapter One", "start": 0.0, "end": 168.0 }
  ],
  "duration_seconds": 36000.0,
  "codec_profile": "mp3",
  "layout": "folder",
  "tracks": [
    {
      "index": 0,
      "url": "/api/media-items/{id}/audio/0",
      "title": "01 First",
      "path": "Author/Title/01 First.mp3",
      "start": 0.0,
      "end": 168.0,
      "duration_seconds": 168.0,
      "size_bytes": 2700000
    }
  ]
}
```

- `layout` is `""` for single-file books (the Batch A model — the whole
  file streams through the `/uploads` path), `"folder"` for a directory
  of track files and `"zip"` for a zip-of-audio book.
- `tracks` is empty for single-file books and carries one entry per
  playable file otherwise. `start`/`end` are cumulative GLOBAL seconds —
  the same coordinate system chapters, bookmarks and
  `audio_offset_seconds` use.

## Error Responses

| Code | Description                     |
| ---- | ------------------------------- |
| 400  | Invalid media item id           |
| 401  | Missing/invalid token           |
| 404  | Book not found                  |
| 500  | audio_metadata failed to parse  |