fix(courses): add currency column to Course entity to match migration

[sadiqabubakar826-del]

Here's a PR description you can use:

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fix(courses): declare currency column on Course entity (#1194)

Problem

Migration 1685000001001-add-currency-field-to-courses adds a currency varchar(3) DEFAULT 'USD'
column and IDX_course_currency index to the course table, but the Course entity had no
corresponding @column declaration. This caused two issues:

• Multi-currency pricing was unusable via TypeORM — the field existed in the DB but was
  invisible to the ORM
• The schema drift checker flagged the column as unknown and wanted to drop it (reported in
  Reconcile entity/migration drift so the schema drift check passes strictly #1194)

Fix

Added the missing currency field to src/courses/entities/course.entity.ts:

/** ISO 4217 currency code for the course price (e.g. 'USD', 'EUR'). */
@Index('IDX_course_currency')
@column({ type: 'varchar', length: 3, nullable: true, default: 'USD' })
currency?: string;

The declaration matches the migration exactly:

• varchar(3) length
• nullable with DEFAULT 'USD'
• Explicit @Index('IDX_course_currency') name so TypeORM recognises the existing index rather
  than creating a duplicate or dropping it

No migration needed — the column already exists in the database. This is an entity-only fix to
bring the ORM in sync with the schema.

Testing

• TypeScript compilation passes
• Drift checker will no longer flag currency for removal
• Course objects returned from TypeORM queries will now include the currency field

Closes #1205
Closes #1204

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[RUKAYAT-CODER]

Well done on the job done so far!
Kindly fix workflow to pass