feat: add content CDN cache invalidation on course update and deletion

[AbuJulaybeeb]

What was done

• Added invalidate(paths: string[]) to CdnService using the @aws-sdk/client-cloudfront CreateInvalidationCommand.
• Integrated opossum Circuit Breaker into CdnService to ensure CDN provider failures do not crash or hang course updates.
• Created CdnEventListener to subscribe to course.updated and course.deleted events and automatically trigger CDN invalidation for the affected paths.
• Wrote unit tests in src/cdn/cdn.service.spec.ts testing the invalidation logic, error handling, and circuit breaker functionality with a mocked CDN provider.
• Registered the new listener within CdnModule.

Why it was done

To resolve issue #844. Previously, students could see stale course materials for hours after a course was updated or deleted because the edge caches weren't invalidated. This implementation ensures real-time cache invalidation on content changes, keeping content served to end-users synchronized.

How it was verified

• Automated unit tests were added verifying that CreateInvalidationCommand receives the correct distribution ID and paths.
• Circuit breaker tests verify that timeouts or failures gracefully fall back and return failure responses instead of propagating errors.

Closes #844

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[RUKAYAT-CODER]

Great job so far

There’s just one blocker — the workflow is failing. Could you take a look and fix it so all checks pass?

Happy to review again once that’s done.

[AbuJulaybeeb]

Great job so far

There’s just one blocker — the workflow is failing. Could you take a look and fix it so all checks pass?

Happy to review again once that’s done.

ohk on it