docs: update documentation for media scanner naming

- Update development.md with new file names (scanner.go, media_scanner.go)
- Update API reference: "Book/ebook operations" -> "Media item operations"
- Remove historical migration comments from schema:
  - Simplify media_items table comment
  - Remove backward compatibility comments for reading_progress and media_ratings
  - Remove note about backward compatibility views
- Remove historical comment from queries.sql about ebook folders

docs/developer/api/api-reference.md

@@ -9,7 +9,7 @@ Complete reference for Bookhoard REST API endpoints.
 - [Admin](admin/) - Admin operations (user management)
 - [Libraries](libraries/) - Library management
 - [Books](books/) - Bulk book operations and downloads
-- [Media Items](media-items/) - Book/ebook operations
+- [Media Items](media-items/) - Media item operations
 - [Progress](progress/) - Reading progress tracking
 - [Notes](notes/) - User notes management
 - [Highlights](highlights/) - Book highlights