Show benchmark shapes in leaderboard description

[msaroufim]

Summary

• Extract benchmarks from the task JSONB column in the leaderboard API
• Display benchmark shapes as a bullet list in the Description section on the leaderboard page
• Seeds are filtered out from the display since they're not useful to participants

Test plan

• Visit a leaderboard with benchmarks (e.g. /leaderboard/697) and verify shapes appear below the description
• Verify seeds are not shown
• Verify leaderboards without benchmarks still render correctly (no empty section)