Add canonical leaderboard rankings API

[msaroufim]

Summary

• add GET /leaderboard/{leaderboard_id}/rankings as the canonical all-GPU leaderboard ranking payload for kernelboard
• return leaderboard metadata plus per-GPU ranked entries using kernelbot's existing get_leaderboard_submissions path
• include submission_count in ranked entries so kernelboard can preserve its current display data
• add a DB helper to resolve leaderboards by id

Why

Kernelboard still has direct SQL for ranking pages. That duplicates rank eligibility semantics outside kernelbot, which is how failed secret runs could still render after kernelbot's API filtering was fixed. This endpoint gives kernelboard a single canonical read path for the main leaderboard page.

Validation

• uv run pytest -q tests/test_admin_api.py::TestClosedLeaderboardAccess::test_public_leaderboard_rankings_no_auth tests/test_admin_api.py::TestClosedLeaderboardAccess::test_missing_leaderboard_rankings_returns_404 tests/test_leaderboard_db.py::test_failed_secret_benchmark_hides_public_leaderboard_score tests/test_leaderboard_db.py::test_leaderboard_submission_count
• uv run python -m py_compile src/kernelbot/api/main.py src/libkernelbot/leaderboard_db.py src/libkernelbot/db_types.py tests/test_admin_api.py tests/test_leaderboard_db.py
• git diff --check

Note: full-file ruff check on the touched kernelbot files is currently noisy with pre-existing import/line-length issues outside this change.

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/libkernelbot
leaderboard_db.py					658-684
utils.py
Project Total

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