core: implement Horizon failover with multiple RPC endpoints

[EmeditWeb]

Problem

The API uses a single hardcoded Horizon endpoint.
If that endpoint goes down, the entire API stops
working.

What To Build

1. Create src/stellar/horizon-client.service.ts
2. Multi-endpoint configuration
3. Circuit breaker per endpoint
4. Health endpoint for monitoring

Files To Touch

• src/stellar/horizon-client.service.ts (new)
• src/stellar/stellar.service.ts
• src/modules/health/health.service.ts

Acceptance Criteria

• Failover happens automatically on primary failure
• Circuit breaker prevents hammering down endpoints
• Health endpoint shows endpoint statuses
• npm run build passes

Mandatory Checks Before PR

• npm run build passes
• No any types
• PR references this issue

[Vicsygold]

BOSS PLS I NEED THIS PROJECT

[Godfrey-Delight]

I've worked with this repo before, so I'm familiar with it. Please let me take this one off your plate as well

[grantfox-oss]

🦊 GrantFox — @Godfrey-Delight has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #27)
2. Your PR will be reviewed by the StepFi-app maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@Godfrey-Delight's PR #86 was approved and merged by @EmeditWeb.

🏆 @Godfrey-Delight: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (764 total points). Track your full progress on GrantFox.

👏 Great work, @Godfrey-Delight! Keep contributing to StepFi-app.