Add container healthchecks for backend, frontend, and worker in staging/production

**Is your feature request related to a problem? Please describe.**

Staging and production app containers (backend, frontend, worker) do not define `healthcheck` blocks, so Docker can’t reliably detect broken app states or surface accurate health for monitoring. Cache/DB health checks exist, but they do not prove the app processes are healthy.

**Describe the solution you'd like**

Add healthchecks for:
- **Backend**: HTTP check against `/status/` (exists and returns 200).
- **Frontend**: HTTP check against `/api/health` (to be added).
- **Worker**: process check (e.g., `pgrep -f rqworker`) or a lightweight Redis ping script.
Apply to both staging and production compose files for environment parity.

**Are you going to work on implementing this?**
- [x] Yes
- [ ] No

**Additional context**
- Backend `/status/` endpoint is defined in [backend/settings/urls.py](https://github.com/OWASP/Nest/blob/main/backend/settings/urls.py#L29).
- Staging and production compose files: [docker-compose/staging/compose.yaml](https://github.com/OWASP/Nest/tree/main/docker-compose/staging), [docker-compose/production/compose.yaml](https://github.com/OWASP/Nest/tree/main/docker-compose/production).