feat(types): expose the book file id on audiobook manifest assets

Clients that download an audiobook get its files from the book detail and its playback order from
the manifest, and had to pair the two by position. The two lists are ordered differently, so an
index pairing can attach the wrong file to an asset. Add the book_files id to each manifest asset
so the pairing can be made by id.

The field is additive and the server always sends it. The manifest revision is hashed from file
and chapter data, not from the response, so existing revisions do not change and clients holding
one do not see a 412 from this deploy.

The iOS app consumes this field to pair downloaded files with manifest assets; the web client
does not read it yet, so a grep of the client alone will show it as unused.

packages/types/src/audiobook.ts

@@ -8,6 +8,8 @@ export const AUDIOBOOK_MANIFEST_VERSION = 2 as const;
 
 export interface AudiobookManifestAsset {
   assetId: string;
+  /** The book file id, matching `BookDetail.files[].id`, so clients can pair an asset with its file. */
+  fileId: number;
   sequence: number;
   format: string;
   durationMs: number | null;