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