feat(api): extend health endpoint with detailed component checks

[observerr411]

This closes #44

Extend GET /api/health to return structured component health status for monitoring and load balancer integration.

• Add database health check using SELECT 1
• Add optional Soroban RPC health check
• Add optional Horizon health check
• Aggregate component results into unified response:
  {
  status: "ok | degraded | down",
  version?: string,
  checks: {
  database: "ok | degraded",
  soroban_rpc?: "ok | degraded",
  horizon?: "ok | degraded"
  }
  }
• Return 200 when all critical components are healthy
• Return 503 when any critical component is down

Improves observability and infrastructure readiness.

[greatest0fallt1me]

@observerr411 Can you resolve the conflicts?