[APIView] WI-13: PR Cleanup via Retention

## Goal

Use the unified retention system for PR version lifecycle instead of ad-hoc `CleanupPullRequestData`.

**Area:** PR Lifecycle · **Depends on:** WI-5, WI-10 · **Stable alone:** ✅ Yes

## Deliverables

### Replace `CleanupPullRequestData` with version-level retention

1. PR transitions to `Merged` (non-promoted): set `RetainUntil = now + MergedPullRequestDays` (default 60d)
2. PR transitions to `Closed`: set `RetainUntil = now + ClosedPullRequestDays` (default 30d)
3. WI-5 purge loop handles actual deletion
4. Promoted PRs: `RetainUntil` cleared (become `Stable`/`Preview`, follow those rules)

Existing `PullRequestBackgroundHostedService` (async PR comment posting) unchanged.

### Tests

- `PullRequestCleanupTests.cs` — Merged non-promoted gets 60d; closed gets 30d; promoted clears `RetainUntil`

Start with PullRequestCleanupTests.cs and trace the PR lifecycle code that currently uses CleanupPullRequestData. Check how merged, closed, and promoted transitions interact with the WI-5 purge loop, while leaving PullRequestBackgroundHostedService unchanged. Done means the tests verify 60-day merged retention, 30-day closed retention, and cleared retention for promoted PRs.

Written by the indexing model from the issue text.