feat: Track approved agents timeline

[jmnmv12]

Summary

This PR updates the GET /api/evaluation-sets/{ID}/approved-agents endpoint to return 3 new fields and order the results so that the newest approved agents come first. Updated response schema:

[
  {
    "id": "3fa85f64-5717-4562-b3fc-2c963f66afa6",
    "miner_hotkey": "string",
    "name": "string",
    "version_num": 0,
    "created_at": "2026-06-22T16:47:20.487Z",
    "final_score": 0,
    "emission": 0,
    "approved_at": "2026-06-22T16:47:20.487Z", --new
    "average_runtime_seconds": 0, --new
    "average_cost_usd": 0 --new
  }
]

It also updates the _should_run_auto_approval_judge to match the /scoring/weights logic. This means that if a candidate agent and the current leader have the same score it will use the average cost as a tie breaker and if that's the same then it will use the agent creation datetime as a tie breaker.