[codex] Add container health check endpoint

[dorakingx]

Summary

• add a lightweight /healthz endpoint for container liveness checks
• wire Dockerfile and docker-compose backend health checks to /healthz
• document the container health-check URL in the README
• add coverage for the new endpoint

Why

Issue #474 asks for a health-check endpoint for container orchestration. The existing /health response is more descriptive, while /healthz gives orchestrators a minimal stable target that only needs to return 200 and { "status": "ok" }.

Validation

• git diff --check
• FastAPI TestClient smoke check for /health, /healthz, and /ping
• Parsed docker-compose.yml and verified the backend healthcheck points at /healthz

Note: running the repo's pytest command locally is blocked by missing native libmagic on this machine (ImportError: failed to find libmagic). The endpoint was still validated with TestClient using a local import stub for the unrelated magic dependency.

Closes #474