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/models.go

@@ -47,6 +47,13 @@ type ReadingProgress struct {
 	LastReadAt  pgtype.Timestamptz `db:"last_read_at" json:"last_read_at"`
 }
 
+type UserEbookFolders struct {
+	ID         pgtype.UUID        `db:"id" json:"id"`
+	UserID     pgtype.UUID        `db:"user_id" json:"user_id"`
+	FolderPath string             `db:"folder_path" json:"folder_path"`
+	CreatedAt  pgtype.Timestamptz `db:"created_at" json:"created_at"`
+}
+
 type Users struct {
 	ID           pgtype.UUID        `db:"id" json:"id"`
 	Email        string             `db:"email" json:"email"`