Use kernelbot leaderboard rankings API

[msaroufim]

Summary

• move /api/leaderboard/<id> ranking data from local SQL to kernelbot's canonical rankings endpoint
• remove duplicated ranked-runs SQL from the API path and legacy server-rendered leaderboard view
• keep kernelboard's existing response transformation so the frontend contract stays stable
• add a focused SQL regression guard so leaderboard ranking routes cannot reintroduce local execute/SQL logic
• fix the local Postgres fixture to preserve PATH when invoking psql

Why

Kernelboard should not own ranking eligibility rules. Kernelbot owns submissions, runs, secret/public validation, and score eligibility. This makes the leaderboard page render kernelbot's canonical view instead of reimplementing rank SQL in the frontend service.

This depends on gpu-mode/kernelbot#493 being deployed first because it introduces GET /leaderboard/{leaderboard_id}/rankings.

Validation

• REDIS_URL=redis://localhost:6380/0 uv run pytest -q tests/api/test_leaderboard_api.py tests/test_leaderboard.py tests/test_leaderboard_sql_guard.py
• REDIS_URL=redis://localhost:6380/0 uv run ruff check kernelboard/api/leaderboard.py kernelboard/leaderboard.py kernelboard/lib/kernelbot_client.py tests/api/test_leaderboard_api.py tests/test_leaderboard.py tests/test_leaderboard_sql_guard.py tests/conftest.py
• REDIS_URL=redis://localhost:6380/0 uv run python -m py_compile kernelboard/api/leaderboard.py kernelboard/leaderboard.py kernelboard/lib/kernelbot_client.py tests/api/test_leaderboard_api.py tests/test_leaderboard.py tests/test_leaderboard_sql_guard.py tests/conftest.py
• git diff --check