feat: update database queries for multiple folder management

- Replace single folder queries with multiple folder operations
- Add AddUserEbookFolder, GetUserEbookFolders, DeleteUserEbookFolder queries
- Regenerate Go code with sqlc
- Update models to include UserEbookFolders struct

backend/internal/database/queries/queries.sql

@@ -96,4 +96,16 @@ WHERE ebook_id = $1 AND user_id = $2
 RETURNING *;
 
 -- name: DeleteEbookRating :exec
-DELETE FROM ebook_ratings WHERE ebook_id = $1 AND user_id = $2;
+DELETE FROM ebook_ratings WHERE ebook_id = $1 AND user_id = $2;
+
+-- name: AddUserEbookFolder :one
+INSERT INTO user_ebook_folders (user_id, folder_path) VALUES ($1, $2) RETURNING *;
+
+-- name: GetUserEbookFolders :many
+SELECT * FROM user_ebook_folders WHERE user_id = $1 ORDER BY created_at;
+
+-- name: DeleteUserEbookFolder :exec
+DELETE FROM user_ebook_folders WHERE user_id = $1 AND folder_path = $2;
+
+-- name: GetEbookByFilePath :one
+SELECT * FROM ebooks WHERE file_path = $1;