fix(miners): sort pull requests by state-relevant timestamp instead of created_at

[Tet-9]

fix(miners): sort pull requests by state-relevant timestamp instead of created_at

Summary

getPullRequests and getPullRequestsByRepo in miners.service.ts sorted all pull request results by p.created_at DESC regardless of PR state. A PR opened 60 days ago but closed 2 days ago would appear near the bottom of results behind PRs opened more recently, making recently-resolved CLOSED PRs appear stale to validators.

Root Cause

Both query variants used a single fixed sort key for all states:

-- Before (incorrect)
ORDER BY p.created_at DESC

-- After (correct)
ORDER BY COALESCE(p.merged_at, p.closed_at, p.created_at) DESC

This is inconsistent with how the filter already distinguishes between states (merged_at for MERGED, closed_at for CLOSED after #139). The sort should reflect the same timestamp hierarchy.

Changes

• packages/das/src/api/miners/miners.service.ts: updated ORDER BY in both getPullRequests (GET path) and getPullRequestsByRepo (POST path) to use COALESCE(p.merged_at, p.closed_at, p.created_at) DESC
• Also fixes the CLOSED filter in both variants to use closed_at >= since consistent with [bug] miners API: CLOSED pull requests filtered by created_at instead of closed_at #139

Testing

Lint confirmed only pre-existing errors in cache.module.ts, unrelated to this change. Verified ORDER BY on issue query variants (lines 278, 321) were not affected.

Fixes #151