Implement email queue dead-letter handling

[hman38705]

Issue 042: Implement email queue dead-letter handling

Area: Backend
Priority: Medium
Files: services/api/src/email/queue.rs

Problem

Jobs that exceed max retries are silently dropped. Failed emails are unrecoverable and unobservable.

Acceptance Criteria

• Permanently failed jobs move to a dead-letter set\n- Dead-letter queue is inspectable via admin API\n- Alerts can be configured on dead-letter growth\n- Manual requeue is supported

[Haroldwonder]

@Haroldwonder has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Please, I would like to work on this

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Congratulations, @Haroldwonder! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @Haroldwonder: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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This issue has been completed by @Haroldwonder as part of the Stellar Wave Program's 4th Wave 🥳

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