refactor(db): add CourseRepository wrapping Course data access (#171)

[Lspnjr1]

Closes #171

Summary

Adds mongo/repositories/CourseRepository.js, extending BaseRepository, so controllers and services stop querying the Course model directly and share one typed data-access surface (consistent pagination, session support, and typed errors).

What's included

Educator listings

• findByEducator(educatorId, options) / paginateByEducator(educatorId, options)

Catalogue discovery

• findPublished(options) — centralized, offset-paginated public catalogue. The Course schema has no draft/published lifecycle field today, so this method is the single place that defines "published"; if a status/isPublished field is later added, only the filter here changes.
• searchCourses(term, options) — full-text search over title/description/category (backed by the model's text index), with a case-insensitive regex fallback for short tokens that $text under-matches.
• findByCategory(categoryRef, options)

Enrollment queries

• findEnrolledCourses(userId, options) / paginateEnrolledCourses(userId, options)
• isUserEnrolled(courseId, userId) — boolean membership check
• countEnrollments(courseId)

Notes

• Every listing supports offset pagination via the inherited paginate(); metadata (total, page, totalPages, hasNextPage…) comes for free.
• ObjectId inputs are validated up front and raise the repository's typed RepositoryValidationError (HTTP 400) instead of silently issuing a query that can never match.
• Purely additive — no existing files modified.

Tests

test/courseRepository.test.js — 17 cases on MongoMemoryServer covering educator listings, catalogue pagination + extra filters, full-text and regex-fallback search, category lookup, enrollment queries, and id validation. All green.

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[Lspnjr1]

Note: this branch also includes a small chore commit that regenerates package-lock.json to match package.json. On dev, the lockfile is missing swagger-ui-express/js-yaml@4 and their @types (added by the API playground PR #277 without regenerating the lock), so npm ci currently fails for every PR to dev. The lock-only regeneration (no runtime dependency changes) unblocks the Run Tests and Syntax-and-Boot-Check jobs. Happy to split it into its own PR if you'd prefer.