docs(api): document the real book download route

The documented GET /api/media-items/:uuid/download is not registered
anywhere - MediaHandler.DownloadBook exists but no route mounts it.
Book files (and covers) are actually served by the JWT-authenticated
GET /uploads/library-{id}/{path} route that the web reader uses.

Rewrite the download doc around the real file route (URL construction
from the item's library_id and relative file_path, MIME/Cache headers,
error codes), note the dead handler so nobody relies on the phantom
endpoint, and correct the API reference index. Mention the OPDS device
route as the conversion-capable alternative.

docs/developer/api/api-reference.md

@@ -89,7 +89,7 @@ See [Media Item Operations](media-items/)
 - GET /api/media-items/:id - Get media item details
 - POST /api/media-items/bulk-delete - Bulk delete media items
 - POST /api/media-items/bulk-update - Bulk update media items (tags/contributors with normalization)
-- GET /api/media-items/:uuid/download - Download media item file
+- GET /uploads/library-{library_id}/{file_path} - Download book file / cover (JWT; see [Download Media Item](media-items/download_media_item.md))
 - POST /api/media-items/:id/rating - Create rating
 - GET /api/media-items/:id/rating - Get rating
 - PUT /api/media-items/:id/rating - Update rating