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

internal/database/queries/queries.sql

@@ -351,9 +351,6 @@ RETURNING *;
 -- name: DeleteMediaRating :exec
 DELETE FROM media_ratings WHERE media_item_id = $1 AND user_id = $2;
 
--- Note: User ebook folders replaced by library folders system
--- Legacy folder management is now handled through libraries
-
 -- Search Media Items queries
 -- name: SearchMediaItems :many
 SELECT mi.*, l.name as library_name, lt.name as library_type_name