feat: add POST /api/v1/admin/courses/:id/archive endpoint

[joelpeace48-cell]

Summary

Adds an admin-only endpoint to archive a course: hides it from public listings while preserving its data so enrolled users keep access and credentials stay intact.

Changes:

• src/database/migrations/0020_courses_archived_at.sql — adds nullable archived_at column to courses
• src/database/schema.ts — adds archivedAt to the courses Drizzle schema
• src/modules/courses/course.service.ts — archiveCourse(courseId): sets isActive = false and archivedAt = now(), invalidates course caches, and writes a course.archived audit log entry (mirrors the existing deleteCourse/publishCourse pattern)
• src/modules/courses/admin-course.controller.ts — archive controller action
• src/modules/courses/admin-course.routes.ts — POST /:id/archive route, admin-guarded like the other admin course routes

Acceptance criteria:

• ✅ Archived course is hidden from public listings (existing idx_courses_is_active / isActive = true listing query already excludes it)
• ✅ Enrolled users can still access the course (enrollment/course-detail lookups aren't gated on isActive, only the public listing is)
• ✅ Archive action is logged via auditLog("course.archived", { courseId })
• ✅ Course data is preserved — this is an update, not a delete

Closes #358
Closes #363
Closes #362
Closes #360

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