[Backend] Implement Dead Letter Queue Processing

[Abd-Standard]

Description

Create a dead-letter queue for notifications that repeatedly fail delivery.

Tasks
Create DLQ storage.
Move failed notifications.
Provide retry tooling.
Add monitoring support.

Acceptance Criteria
Failed notifications enter DLQ.
Retry actions are supported.
Monitoring identifies DLQ growth.

Contributors start work by running:

git checkout -b backend/dead-letter-queue

[Banx17]

@Banx17 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello Team I'd love to be assigned this issue. As a backend developer, I've experience across many frameworks, and I'm confident I'll be able to resolve this. Looking forward to it. Thanks

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Banx17 to this issue.

[grantfox-oss]

🦊 GrantFox — @francisdouglas-ux 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 #286)
2. Your PR will be reviewed by the Core-Foundry maintainers

Good luck! Track your progress on GrantFox.

[drips-wave]

Congratulations, @francisdouglas-ux! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @francisdouglas-ux: 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

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[serverlessdomain-hash]

@serverlessdomain-hash has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I'm a full-stack developer and I'd like to take this issue.
I understand the requirements and will follow the existing project structure, keep the implementation clean, and include proper tests where applicable.

ETA: 1–3 days (depending on scope)

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

[drips-wave]

Congratulations, @serverlessdomain-hash! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @serverlessdomain-hash: 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

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[Olajcodes]

@Olajcodes has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, kindly assign this issue to me. I have worked on issue like this before. I believe I'm capable.

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