Backend: include prCreatedAt and closedAt on /miners/:id/prs

[Khaostica]

Summary

The das API endpoint GET /miners/:id/prs returns mergedAt but does not currently include prCreatedAt or closedAt, even though both fields are declared on the CommitLog type (src/api/models/Dashboard.ts) and consumed by the UI. Please add these fields to the response.

Motivation

MinerPRsTable renders a Date column whose value depends on PR state — merge date for merged, close date for closed, creation date for open. Without prCreatedAt and closedAt on the wire, non-merged rows currently fall back to — (related PR #1119 — review discussion settled on awaiting these backend fields rather than working around them client-side).

Other code paths in this repo already reference both fields (src/pages/dashboard/dashboardData.ts, src/pages/HomePage.tsx, src/pages/WatchlistPage.tsx), so adding them keeps the wire response consistent with how the rest of the codebase treats CommitLog. Once the fields land, no further UI change is needed — the renderer picks them up automatically.

Suggested response shape

{
  "pullRequestNumber": 994,
  "repository": "entrius/gittensor-ui",
  "prState": "CLOSED",
  "mergedAt": null,
  "closedAt": "2026-05-08T16:59:20.000Z",   // new
  "prCreatedAt": "2026-05-08T16:53:33.000Z" // new
  // ...existing fields
}

Both nullable. prCreatedAt is always present in practice (every PR has a creation timestamp); closedAt is null for open PRs and populated for closed/merged.