Show LOC for public solutions

[msaroufim]

Summary

• Add optional line_count to /api/leaderboard ranking rows only when the leaderboard is concluded and public solution code is allowed.
• Show LOC directly in each ranking row when line_count is present, next to the existing ranking metadata.

Validation

• git diff --check
• .venv/bin/ruff check kernelboard/api/leaderboard.py
• npm run lint
• npm run build

Notes

• Backend route pytest still requires Docker for the Postgres/Redis fixtures; Docker is not running in this local environment.