Add chain-sync lag value to health check payload

[Chucks1093]

Summary
Add chain-sync lag value to health check payload

Scope

• include latest indexed ledger and observed head ledger\n- compute sync lag and include in health response\n- mark degraded status when lag exceeds threshold

Acceptance Criteria

• scoped change is implemented with no unrelated file churn
• docs/tests are updated where applicable
• contributor can validate the outcome locally

Contributor coordination: if you pick this up, reach out in Telegram for updates or clarification: https://t.me/accesslayerorg

[veemakama]

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

Hi, I'd like to work on this issue.
I have experience building observability and health-check systems for distributed and blockchain-integrated services.
I can extend the health check payload to include a chain-sync lag metric, calculated from the difference between local and network head states.
My approach ensures better visibility into sync health, faster issue detection, and more actionable monitoring insights.

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

[logantalen]

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

I'd like to work on this.

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

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

🧑‍💻 @logantalen: 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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[drips-wave]

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

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