docs: update cross-references from /api/books/ to /api/media-items/

- Update api-reference.md with new endpoint paths
- Update api-reference.md Books API section → Media Items API section
- Update index.md Books API link → Media Items API
- Update get_shelf.md cover_url reference from /api/books/ to /api/media-items/

docs/developer/api/api-reference.md

@@ -73,14 +73,6 @@ See [Library Management](libraries/)
 - GET /api/libraries/visibility - Get visible libraries
 - POST /api/libraries/visibility - Set library visibility
 
-## Books
-
-See [Book Operations](books/)
-
-- POST /api/books/bulk-delete - Bulk delete books
-- POST /api/books/bulk-update - Bulk update books (tags/contributors with normalization)
-- GET /api/books/:uuid/download - Download book file
-
 ## Media Items
 
 See [Media Item Operations](media-items/)
@@ -88,6 +80,9 @@ See [Media Item Operations](media-items/)
 - GET /api/media-items - List media items
 - GET /api/media-items/filtered - Filter and sort 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
 - POST /api/media-items/:id/rating - Create rating
 - GET /api/media-items/:id/rating - Get rating
 - PUT /api/media-items/:id/rating - Update rating