feat: add closed_at field to pull request storage

[statxc]

gh pr create --repo entrius/gittensor --base test --head statxc:feat/add-closed-at-to-pull-requests --title "feat: add closed_at field to pull request storage for 35-day window filtering" --body "$(cat <<'EOF'

Summary

• Add closed_at field to PullRequest dataclass in classes.py
• Extract closedAt from GraphQL response in from_graphql_response()
• Add closed_at column to BULK_UPSERT_PULL_REQUESTS INSERT and ON CONFLICT UPDATE in queries.py
• Add pr.closed_at to the values tuple in repository.py

Related Issues

Fixes #396

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other

Testing

• All 272 existing tests pass
• Ruff lint clean
• Pyright typecheck clean (0 errors)
• Manual verification: PullRequest.from_graphql_response() correctly parses closedAt into closed_at

Note: The pull_requests table requires an ALTER TABLE to add the closed_at column:

ALTER TABLE pull_requests ADD COLUMN IF NOT EXISTS closed_at TIMESTAMPTZ;