feat: block certificate access during proctoring review

[santhosh-apphelix-2u]

Summary

Implement end-to-end certificate access blocking for learners whose required proctored exams are incomplete, under review, or in a blocking status.
Ticket - LP-592

Changes

• Add centralized proctoring status evaluation.
• Support the approved blocking and allowed status matrix.
• Treat eligible as derived not_attempted.
• Add the certificate proctoring review feature flag.
• Remove certificate URLs from blocked API and dashboard responses.
• Protect HTML certificate access.
• Add authenticated PDF download handling with live proctoring checks.
• Add fail-closed behavior, request-level caching, and monitoring metrics.
• Add backend tests for statuses, API responses, PDF access, and cross-user security.

Scope

Certificate generation, certificate requesting, and provider-specific proctoring integrations are unchanged.