Harden external API clients with timeouts and retries

[robertocarlous]

Body:

Calls to Twilio, Stellar, and Anthropic lack consistent timeouts and retry behavior. Wrap these clients with a shared HTTP client or adapter that implements timeouts, exponential backoff, and a circuit breaker for repeated failures.

Acceptance Criteria:

• External client wrappers implement timeouts and retry logic.
• Circuit breaker prevents calls after a configurable failure threshold.
• Integration tests simulate failures to confirm fallback behavior.