Add circuit breaker for Hiya/Truecaller external APIs (FRE-4508)

- Implement CircuitBreaker class with CLOSED/OPEN/HALF_OPEN states
- Configurable failure threshold, success threshold, and timeout
- Fallback behavior when circuit opens (returns neutral 0.5 score)
- State change callbacks for monitoring and logging
- Comprehensive metrics tracking (executions, failures, successes, timestamps)
- Update SpamShieldService to use circuit breakers for both Hiya and Truecaller
- Add parallel API calls with graceful degradation
- Export circuit breaker types and service interfaces
- 32 unit tests covering circuit transitions, fallback, and service integration

Co-Authored-By: Paperclip <noreply@paperclip.ing>

services/spamshield/src/index.ts

@@ -0,0 +1,5 @@
+export { SpamShieldService } from './services/spamshield.service';
+export type { ReputationResult, CircuitMetrics } from './services/spamshield.service';
+export { spamRateLimits, spamFeatureFlags, spamConfig } from './config/spamshield.config';
+export { CircuitBreaker, CircuitBreakerError } from './circuit-breaker';
+export type { CircuitState, CircuitBreakerMetrics, CircuitBreakerOptions } from './circuit-breaker';