feat(api): outbox delivery latency metrics and alerting thresholds

[0xDeon]

Instrument internal/domain/outbox to record time-to-first-attempt and time-to-delivery as metrics, so growing backlog or delivery delay is observable before it becomes an incident.

[Uhiara]

@Uhiara has applied to work on this issue as part of the Stellar Wave Program's 9th wave.

Hi maintainer. I would love to work on this issue.

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[Johnalex-hub]

@Johnalex-hub has applied to work on this issue as part of the Stellar Wave Program's 9th wave.

Hello there team, my name is John can i please work on this issue?

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[drips-wave]

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

🧑‍💻 @Johnalex-hub: 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.

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox!

@Johnalex-hub's PR #1368 was approved and merged by @0xDeon.

🏆 @Johnalex-hub: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (153 total points). Track your full progress on GrantFox.

👏 Great work, @Johnalex-hub! Keep contributing to Suncrest-Labs.

[drips-wave]

This issue has been completed by @Johnalex-hub as part of the Stellar Wave Program's 9th Wave 🥳

😎 @Johnalex-hub: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

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