[Notifications] Settlement failure alert to merchant and admin

[Xaxxoo]

Overview

Immediate alert when a settlement fails permanently.

Tasks

• Template for merchant: failure reason, payment reference, retry link
• Separate template for admin: same data plus partner error detail
• Both sent within 1 minute of permanent failure
• Admin alert also goes to Slack (if configured)

Acceptance Criteria

• Merchant email sent immediately on permanent failure
• Admin notified simultaneously
• Failure reason included in both emails

[onahiOMOTI]

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

hi maintainer,
I'd like to work on this issue.

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

[drips-wave]

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

🧑‍💻 @onahiOMOTI: 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 🌊