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

[Yurii214]

Summary

• Fixes [bug] miners API: pull requests sorted by created_at instead of state-relevant timestamp #186: miner pull request listings now sort by the timestamp relevant to each PR state (merged_at for MERGED, closed_at for CLOSED, created_at for OPEN).
• Applies the same state-relevant timestamp to the CLOSED branch of the since window filter (was incorrectly using created_at).
• Aligns keyset pagination cursors with the sort key on both GET /miners/:id/pulls and POST /miners/:id/pulls (per-repo path), so paginated pages stay ordered consistently.

Shared SQL fragments (PR_SORT_AT, PR_ORDER_BY, PR_SINCE_WINDOW*) keep the GET and POST paths identical.

Why

Validators window PRs by since and expect recently merged/closed work to rank above older OPEN PRs. Sorting everything by created_at made stale-looking responses when a CLOSED PR had an old created_at but a recent closed_at.

What might break

• On-disk / API shape: none — response fields unchanged.
• Pagination cursors: cursors issued before this change used created_at as the page key; clients mid-pagination may need to restart from the first page once. New cursors encode the state-relevant sort timestamp in the existing cursor field.

Type of Change

• Bug fix

Testing

• npm run build in packages/das
• npm run lint in packages/das
• npm run format:check in packages/das
• Manual repro: CLOSED PR with old created_at and recent closed_at now sorts above older OPEN PRs when both are in the scoring window

Fixes #186

[Yurii214]

Hi @anderdc — thanks for looking at this. Could you share why #194 was closed?
The sort/pagination fix for #186 isn’t on test yet, and I want to adjust
approach rather than resubmit the same thing. Happy to scope down or wait if
you’re handling #186 internally. Thanks.