feat(stellar): add Horizon circuit breaker

[Gezziy]

closes #372
Summary of changes:

• Added a lightweight circuit breaker for Stellar Horizon network calls.
• Wrapped bounded Horizon calls in StellarService, including account loads, transaction lookups, operations, claimable balances, fee stats, and transaction submission.
• Reused the existing Stellar retry classifier so only transient Horizon/network failures trip the breaker.
• Added focused unit coverage for circuit breaker state transitions and StellarService fail-fast behavior.
• Documented circuit breaker configuration in the backend environment example.

Reason for the changes:
A Stellar Horizon outage or sustained rate limiting could previously cause repeated network calls and cascading backend failures. The circuit breaker fails fast after repeated transient failures, protects backend resources, and allows recovery through a half-open probe after the reset timeout.

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