feat(backend): implement add /health/smoke deployment readiness endpoint

[Neziahtech]

Closes #720

Summary

Adds a GET /health/smoke endpoint to the Express backend that CI pipelines and Vercel preview checks can hit to confirm the backend is alive, all required environment variables are present, and testnet contract IDs are reachable — without exposing any secret values in the response.

What Changed

• Added GET /health/smoke route in apps/backend-api
• Validates presence of required env vars (STELLAR_NETWORK, SOROBAN_RPC_URL, SOROBAN_CONTRACT_ID, PORT, etc.) — returns missing key names only, never values
• Performs a lightweight reachability ping against the configured Soroban RPC URL to confirm the testnet dependency is reachable
• Returns a structured JSON response with an overall status of "ok" or "degraded", per-check results, and a UTC timestamp
• Returns HTTP 200 when all checks pass, 503 when any check fails — making it CI-friendly for exit-code-based gating

Response Shape

{
  "status": "ok",
  "timestamp": "2026-06-28T10:00:00.000Z",
  "checks": {
    "env": { "status": "ok", "missing": [] },
    "soroban_rpc": { "status": "ok", "latency_ms": 142 }
  }
}

On failure:

{
  "status": "degraded",
  "timestamp": "2026-06-28T10:00:00.000Z",
  "checks": {
    "env": { "status": "fail", "missing": ["SOROBAN_CONTRACT_ID"] },
    "soroban_rpc": { "status": "fail", "error": "Connection refused" }
  }
}


## Safety

- No secret values are ever included in the response — only key names for missing vars
- No auth required (intentionally public for CI/Vercel use)
- No internal stack traces or file paths exposed in error messages


## Testing

- [ ] `GET /health/smoke` returns `200` with all env vars set and testnet reachable
- [ ] Returns `503` + `"degraded"` when an env var is missing
- [ ] Returns `503` + `"degraded"` when the Soroban RPC URL is unreachable
- [ ] Verified no secret values appear in any response payload
- [ ] CI workflow updated to curl this endpoint post-deploy as a gate


## Related

- Part of the broader observability/deployment-readiness initiative alongside the existing Prometheus + Grafana setup (`docker-compose.monitoring.yml`)

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[Cedarich]

@Neziahtech please fix workflow