Implement Web3 RPC Fallback & Retry Strategy

[Olowodarey]

Blockchain infrastructure can be flaky. If our primary Stellar Horizon RPC node goes down during a critical payment execution, our backend must failover seamlessly.

Acceptance Criteria

• Update StellarService to maintain an ordered array of acceptable RPC endpoints.
• Intercept network failures using a custom generalized wrapper or interceptor.
• Automatically retry the request using the next available RPC node in the pool.
• Log severe failover events so DevOps is alerted.