Create Health Check and Monitoring Endpoints

[Mathews-25]

Context:
For 99.9% uptime requirement, we need comprehensive health monitoring of all critical services (database, Redis, Stellar network).

Problem:
Implement health check endpoints that verify all external dependencies are operational.

What Done Looks Like:

• /health endpoint returning overall status
• /health/stellar checking Horizon connectivity
• /health/soroban checking Soroban RPC
• /health/db checking PostgreSQL
• /health/cache checking Redis
• Prometheus metrics endpoint

Folder Structure:

src/
├── health/
│   ├── health.controller.ts
│   ├── health.module.ts
│   └── indicators/
│       ├── stellar.health.ts
│       ├── soroban.health.ts
│       └── database.health.ts

Implementation Guidelines:

• Use @nestjs/terminus for health checks
• Return proper HTTP status codes (200 for healthy, 503 for unhealthy)
• Include response time metrics
• Test actual connectivity, not just service existence
• Add custom health indicators for Stellar-specific checks

Response Format:

{
  "status": "ok",
  "info": {
    "database": { "status": "up" },
    "redis": { "status": "up" },
    "stellar": { "status": "up", "latency": "45ms" },
    "soroban": { "status": "up", "latency": "120ms" }
  }
}

Validation:

• All health checks pass in healthy state
• Returns 503 when service is down
• Response time under 1 second

---