feat(risk-management): production-ready circuit breaker & health checks

[BigBen-7]

Summary

Closes #11

This PR aligns the investment/risk-management module to production standards by replacing the minimal circuit breaker stub with a full implementation backed by @nestjs/terminus health reporting and EventEmitter2 event emission.

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What changed

CircuitBreakerService (rewrite)

• Three-state machine: CLOSED → OPEN → HALF_OPEN → CLOSED with clean transition logging.
• Configurable thresholds per protected service via configure(serviceName, config):
  • failureThreshold — number of failures before opening (default: 5)
  • slowCallDurationMs — duration that qualifies a call as slow (default: 5 000 ms)
  • slowCallRateThreshold — % of slow calls that trips the breaker (default: 50 %)
  • recoveryTimeMs — time before transitioning to HALF_OPEN (default: 60 s)
• In-memory metrics: failure count, success count, slow-call count, total call count, last failure time, last transition time — all queryable per service.
• Half-open probe: after recovery time the next isOpen() call transitions to HALF_OPEN. One successful recordSuccess() closes the circuit; one recordFailure() reopens it.
• Events emitted on every state transition:
  • circuit-breaker.opened { serviceName, failureCount, timestamp }
  • circuit-breaker.closed { serviceName, timestamp }
• Per-service isolation — multiple services can each have their own breaker state and config.

RiskManagementService

• Injected EventEmitter2 and emits risk.threshold.breached { userId, alert } for every alert produced by calculatePortfolioRisk, enabling the alerts module to subscribe without coupling.

RiskManagementHealthIndicator (new)

• Extends HealthIndicator from @nestjs/terminus.
• isHealthy('risk-management') reports healthy when the circuit is CLOSED or HALF_OPEN, unhealthy (throws HealthCheckError) when OPEN.

HealthModule / HealthController (new — src/health/)

• TerminusModule + RiskManagementModule imported.
• GET /api/v1/health now returns a proper terminus health-check response including the risk-management indicator.
• Replaced the old basic { status, timestamp } handler in AppController.

RiskManagementModule

• Imports EventEmitterModule.forRoot().
• Provides and exports RiskManagementHealthIndicator.

AppModule

• Registers EventEmitterModule.forRoot() globally.
• Imports HealthModule.

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Tests

19 unit tests in circuit-breaker.service.spec.ts covering:

• CLOSED state: initial state, threshold not reached, success metrics, isOpen returns false.
• OPEN state: threshold breach, circuit-breaker.opened event, auto-transition to HALF_OPEN after recovery time.
• HALF_OPEN state: successful probe → CLOSED + circuit-breaker.closed event; failed probe → OPEN + circuit-breaker.opened event.
• reset: counters cleared, emits circuit-breaker.closed only when previously non-CLOSED.
• Per-service config: custom threshold respected, services isolated from each other.

Test plan

• npm run build — webpack compiled successfully ✅
• npx jest --testPathPattern="circuit-breaker" — 19/19 passed ✅
• GET /api/v1/health returns terminus JSON with risk-management status
• Calling POST /api/v1/risk/circuit-breaker/reset still works as before
• GET /api/v1/risk/circuit-breaker returns the new richer status shape