[bug] miners API: pull requests sorted by created_at instead of state-relevant timestamp

[kiannidev]

Description

GET/POST /api/v1/miners/:githubId/pulls always sorts results by p.created_at DESC. For MERGED and CLOSED PRs that enter the scoring window via merged_at / closed_at, this ranks recently merged/closed work below older OPEN PRs and makes validator responses look stale.

Steps to Reproduce

1. Mirror two PRs by the same miner in the same repo:
   • PR A: OPEN, created_at = 2 days ago
   • PR B: CLOSED, created_at = 60 days ago, closed_at = 1 day ago
2. Call GET /api/v1/miners/{id}/pulls?since= (30 days ago).
3. PR A appears before PR B even though B closed more recently.

Expected Behavior

Sort by the timestamp relevant to each state: created_at for OPEN, merged_at for MERGED, closed_at for CLOSED.

Actual Behavior

ORDER BY p.created_at DESC

Environment

• OS: Linux
• Runtime/Node version: Node 20