[P1][backend] Add health checks for PostgreSQL, Redis, and Stellar RPC

[dzekojohn4]

Expose readiness and liveness checks separately. Ensure readiness fails when required dependencies are unavailable and document probe endpoints.

[okoye-collins]

@okoye-collins has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

hey, maintainer, I will take up this issue

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[gbengaeben]

@gbengaeben has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello Maintainer
I would love to work on this issue please kindly assign

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[drips-wave]

Congratulations, @gbengaeben! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @gbengaeben: 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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