Add timeout and circuit breaker for external blockchain RPC calls

[Smartdevs17]

Description

External RPC calls have no timeout causing cascading failures. Implement circuit breaker pattern.

Acceptance Criteria

• Configurable timeout per RPC endpoint
• Circuit breaker (failure threshold, recovery timeout)
• RPC provider fallback on failure
• Circuit state monitoring dashboard
• Graceful degradation on circuit open
• Manual circuit reset

Technical Scope

• backend/services/
• Edge: RPC provider diversity, fallback consistency, partial response handling