feat(health): BA-112 report dependency health separately from application health

[ranjeetprabhatedu-dotcom]

Summary

This PR implements BA-112 to report external dependency health separately from core application health, ensuring graceful operational degradation without false-positive core crashes.

• Updates HealthController in health.controller.ts with checkDependencies() inspecting database, Stellar Horizon/RPC, and storage health indicators.
• Distinguishes core application uptime (application.status: 'up') from external dependency failures (status: 'degraded').
• Sets HTTP 503 SERVICE_UNAVAILABLE when critical dependencies are degraded while preserving diagnostic payload body.
• Fixes exception import in coverage.ts to use HttpException with HttpStatus.TOO_MANY_REQUESTS.
• Adds unit tests in app.controller.spec.ts covering healthy baseline state, isolated database degradation, and isolated Stellar RPC degradation.
• Scoped strictly to the 3 allowlisted files:
  • BackendAcademy/src/health/health.controller.ts
  • BackendAcademy/src/health/coverage.ts
  • BackendAcademy/src/app.controller.spec.ts

Closes #1025