course entity has no currency column but a migration adds one

[RUKAYAT-CODER]

Migration 1685000001001-add-currency-field-to-courses.ts adds currency varchar(3) DEFAULT 'USD' (plus index IDX_course_currency) to the course table, but Course (src/courses/entities/course.entity.ts) has no currency column.

The app can't use multi-currency pricing via TypeORM, and the drift check wants to drop the column (see #1194).

Fix: declare currency on the Course entity with the same type/default, and align the index name (IDX_course_currency).

[Meet-hybrid]

I would like to contribute to this issue. I have experience with JavaScript/TypeScript, React, backend APIs, testing, Git, and software architecture. I will follow the acceptance criteria, add tests, and provide a focused pull request.