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.

Vom Indexierungsmodell aus dem Issue-Text verfasst.