core: implement Horizon failover with multiple RPC endpoints

[Godfrey-Delight]

🔗 Related Issue

Closes #27

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🔖 Title

Created src/stellar/horizon-client.service.ts with multi-endpoint support (configurable via STELLAR_HORIZON_URLS env var, comma-separated), a circuit breaker per endpoint (3 failures -> 30s cooldown -> half-open probe -> automatic recovery), and auto-failover to the next healthy endpoint on transient failures (timeouts, 5xx, rate limits, network errors). Created src/stellar/stellar.service.ts as a thin public-facing wrapper. Updated src/modules/health/health.service.ts to consume the horizon client and report per-endpoint circuit breaker statuses (closed/open/half-open, failure count, last error, primary indicator) in the GET /health response. Two new config keys are supported: STELLAR_HORIZON_URLS (comma-separated list) with fallback to the existing STELLAR_HORIZON_URL.

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📝 Description

Replaces the single hardcoded Horizon endpoint with a multi-endpoint client that fails over automatically when the primary goes down, so the API doesn't stop working on a single endpoint outage. Adds a circuit breaker per endpoint to stop hammering a failing one, and surfaces per-endpoint health status through the existing /health route for monitoring.

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🔄 Changes Made

• Added src/stellar/horizon-client.service.ts — multi-endpoint client with per-endpoint circuit breakers (3 failures → 30s cooldown → half-open probe → auto-recovery) and failover on timeouts, 5xx, rate limits, and network errors
  -Added src/stellar/stellar.service.ts as a thin wrapper exposing the horizon client to the rest of the app
• Updated src/modules/health/health.service.ts to report per-endpoint status (closed/open/half-open, failure count, last error, which one is primary) in GET /health
• Added STELLAR_HORIZON_URLS (comma-separated) config, falling back to the existing STELLAR_HORIZON_URL for backward compatibility

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📸 Screenshots (if applicable)

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🗒️ Additional Notes

[EmeditWeb]

Review verdict: ✅ APPROVE

Clean multi-RPC failover. withFailover<T> rotates through all configured endpoints, tries each on failure, and throws only once every endpoint is exhausted; endpoints come from env with a sensible testnet default. No any; 21 tests pass locally; CI green.

One note: submitTransaction also goes through withFailover, so if the first endpoint accepts the transaction but its response is lost, a retry could re-submit to a second endpoint. For Stellar this is largely idempotent (the same signed tx / sequence number yields the same result or tx_bad_seq), so it's low-risk — but a short code comment noting that submit-retries rely on ledger-level idempotency would be worth adding. Otherwise merge-ready.