Add healthz endpoints and container healthchecks

[pgilad]

Summary

This PR adds lightweight health endpoints and container healthchecks for both services so Docker/Compose can reliably detect service health and gate startup order.

Changes

• Added backend health endpoint:
  • GET /healthz returns 200 OK with ok.
• Added frontend health endpoint:
  • GET /healthz returns 200 OK with ok.
• Added backend healthcheck script:
  • backend/server/healthcheck.py
  • Uses Python stdlib HTTP client (no curl/wget dependency).
• Added frontend healthcheck script:
  • frontend/healthcheck.mjs
  • Uses Node fetch (no extra packages).
• Added image-level Docker HEALTHCHECK directives:
  • backend/Dockerfile
  • frontend/Dockerfile
• Added Compose-level healthchecks and startup gating:
  • docker-compose.yml
  • docker-compose.dev.yml
  • docker-compose-traefik.yaml
  • web now depends on server with condition: service_healthy.

Why

• Improves container health visibility (healthy/unhealthy).
• Avoids shipping extra network tools in runtime images.
• Prevents frontend from starting before backend is actually healthy in Compose examples.

Validation

• Python syntax checks passed for new/updated backend files.
• Node syntax check passed for frontend/healthcheck.mjs.
• Verified new route and healthcheck wiring is present across Dockerfiles and compose files.

Notes

• Existing prebuilt images must include these new files; otherwise healthchecks that reference them will fail until images are rebuilt/pulled with this change.

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