fix(backfill): use baseRefName so deleted base branches don't null out base_ref

[anderdc]

Problem

The GraphQL PR backfill selects baseRef { name }. In GitHub's schema baseRef is a nullable Ref that resolves to null once the base branch is deleted or renamed away. Since the persist is an upsert keyed on (repoFullName, prNumber), the nightly backfill overwrites a previously-correct webhook-written base_ref with null — pure data loss for the validator's branch-eligibility gate.

Fix

Select baseRefName (non-nullable String!) instead; it preserves the historical base ref name on the PR record even after branch deletion. baseRefOid and headRef are untouched.

Existing null rows self-repair whenever the incremental backfill next touches them.