Add email delivery retry and bounce handling

[nanaf6203-bit]

Retry failed email deliveries and handle bounces.

Acceptance Criteria:

• Failed email sends are retried with backoff.
• Bounce events are recorded and processed.
• Permanent failures update user preferences appropriately.
• Tests cover retry and bounce handling.

[AbuJulaybeeb]

@AbuJulaybeeb has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi Maintainer
I would like to hop on this issue

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

[Deeeelighttt]

@Deeeelighttt has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi, I’m a software engineer with experience working on similar features, and I’m interested in contributing to this project. I’ve reviewed the issue and have a good understanding of what’s required to implement the proposed solution.

I’d be happy to take this on and submit a pull request. Could you please assign this issue to me?

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

[drips-wave]

Congratulations, @Deeeelighttt! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @Deeeelighttt: 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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