[Backend] Cron: Mark expired payments and emit webhook

[JayWebtech]

Overview

Pending payments past expiration should transition to expired and notify merchants.

Technical Requirements

• Scheduled job to find pending + expiration < now.
• Update status; send payment_failed or payment_expired event per product spec.
• Idempotent updates.

[GideonBature]

@GideonBature has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Greetings Maintainer,

Please I will love to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @GideonBature to this issue.