[Contracts] Implement credential expiration with automatic status transitions

[jobbykings]

Description

Credentials currently have no expiration concept. Many educational credentials (certifications, licenses) should expire after a set period and require renewal. We need expiration timestamps on credentials with on-chain status that automatically reflects expiration.

Definition of Done

• Optional expires_at: Option<u64> field added to the credential struct in credentials.rs
• issue_credential accepts optional expiration timestamp; if None, credential never expires
• Credential status enum updated: Active, Revoked (from related issue), Expired
• verify_credential checks expiration: if expires_at < current_ledger_timestamp, returns Expired status
• No automatic state change transaction required — expiration is checked at read time (lazy evaluation for gas efficiency)
• get_credential_status(id) function that returns (Active/Expired/Revoked) and the expired_at timestamp if applicable
• Renewal: renew_credential(id, new_expires_at) function callable by original issuer, updates expiration
• Renewal emits CredentialRenewed event
• Storage optimization: expiration stored as optional packed timestamp
• Unit tests: issue with expiration, issue without expiration, verify after expiration, renew credential, renew non-expired, unauthorized renewal

Files to Modify

• contracts/src/credentials.rs (add expires_at field, CredentialStatus enum, renew function)
• contracts/src/credential_registry.rs (integrate expiration in issue/verify flow)
• contracts/src/credentials_test.rs (or test module)
• contracts/src/event_logger.rs (add CredentialRenewed event)

Acceptance Criteria

• Credential issued with expires_at = block_ts + 1_year shows Active when verified within 1 year
• Same credential shows Expired when verified after 1 year
• Credential issued without expiration shows Active indefinitely
• renew_credential(id, block_ts + 2_years) extends expiration; verification now shows Active again
• Non-issuer calling renew_credential fails with authorization error
• Renewing a revoked credential fails (revoked cannot be renewed)
• CredentialRenewed event includes credential ID, old expiration, new expiration
• Gas benchmark for verify_credential with and without expiration check

[Tobi-8]

Please maintainer assign me to this issue
I have the knowledge and skill to complete this task

[Kami-no-san]

hi i would like to work on this

[grantfox-oss]

🦊 GrantFox — @Kami-no-san has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #128)
2. Your PR will be reviewed by the Epondia maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@Kami-no-san's PR #140 was approved and merged by @jobbykings.

🏆 @Kami-no-san: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (853 total points). Track your full progress on GrantFox.

👏 Great work, @Kami-no-san! Keep contributing to Epondia.