EduVault buyer entitlement checks need stale-index fallback before granting downloads

[prissca]

Problem

A cached entitlement can become stale after refund, revoke, or indexer lag, so download access must fail safely when source-of-truth status is uncertain.

Scope

• Reconcile cached entitlement with latest purchase state
• Return pending/denied states when indexer freshness is insufficient
• Add tests for delayed and missing purchase events

Acceptance criteria

• Downloads are not granted from stale entitlement cache alone
• Pending indexer state is visible to users
• Tests cover purchase, refund, revoke, and indexer lag

Relevant areas

• docs/entitlement-authorization.md
• docs/purchase-flow-architecture.md
• src

Complexity

High - 200 points

Contributor guidance

Start with a short design note in the issue discussion, then implement with focused tests and documentation for the affected workflow.