B2B-13 · Real health checks (terminus: Mongo + Kratos, readiness/liveness)

Part of the B2B epic (Bloco 3 · item 3.4). Quick win, fully independent. Today `server/root.controller.ts:17-20` returns hardcoded `{status:"ok"}`; `@nestjs/terminus` is not a dependency.

## What
- [ ] Add `@nestjs/terminus`
- [ ] Mongo + Kratos health indicators
- [ ] Readiness/liveness endpoints (wire to k8s probes in `deployment/`)

**Effort:** ~0.5 SP · **Depends on:** —

Start at server/root.controller.ts:17-20 to inspect the current hardcoded health response, then inspect deployment/ for the Kubernetes probe configuration. Add the @nestjs/terminus dependency and Mongo and Kratos health indicators, expose readiness and liveness endpoints, and verify that the deployment probes target them.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.