fix(indexer): add DB, Redis, and cursor checks to the indexer health server (currently only RabbitMQ)

[cypriannwokolo2-creator]

Summary

cmd/indexer/main.go:135 health server checks only RabbitMQ, not DB/Redis/cursor freshness — an unhealthy indexer can look healthy.

Acceptance Criteria

• Health reflects DB/Redis/cursor lag
• Metrics for cursor lag
• Tests

Complexity

Medium-High.

[deslawson]

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

I can handle this issue

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[AnnieIj]

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

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

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

[danfulani0]

@danfulani0 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer! I'm a senior developer with experience building high-quality, production-ready software. I'd love to take on this issue and deliver a clean, well-tested solution that aligns with the project's standards. Please assign it to me—I'm ready to get started and contribute effectively.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @danfulani0 to this issue.