fix(backfill): create issue rows before PR metadata jobs

[JSONbored]

Summary

• Move issue backfill before PR metadata/file job enqueueing during repository backfill.
• Keep the change scoped to the existing backfill ordering path.

What changed

• handleBackfill() now calls backfillIssues() immediately after PR backfill completes.
• PR metadata and file follow-up jobs are enqueued only after issue rows have been fetched/upserted.

Why

PR metadata jobs can only link solved_by_pr when the referenced issue rows already exist. Enqueueing metadata jobs before issue backfill can leave already-seen PRs without issue attribution during backfill.

Fixes #28

Validation

• npm ci
• npm run build
• npm run lint
• npm run format:check
• git diff --check