fix: enqueue PR files refresh on base-branch edits

[jakearmstrong59]

Summary

The pull_request webhook handler enqueues a PR_FILES job on opened, synchronize, and merged closed, but not on edited. When a maintainer retargets a PR's base branch, GitHub fires pull_request.edited with changes.base populated and a new pr.base.sha. Today the handler writes the new baseSha onto the PR row but leaves pr_files, pr_file_contents, and merge_base_sha pinned to the previous base — and scoring_data_stored stays true, falsely advertising the bundle as current.

This change detects base retargets on pull_request.edited (via payload.changes?.base), clears scoring_data_stored, and enqueues the same PR_FILES job the synchronize path uses. The job's deterministic prFilesJobId(repo, pr, headSha, baseSha) naturally dedupes against any in-flight fetch for the new generation, and fetchAndStorePrFiles already
recomputes the merge-base and re-upserts file content on every run, so no fetcher changes are required.

Title/body edits are untouched — they don't set changes.base, so they keep running only the existing PR_METADATA job.

Related Issues

Fixes #62

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

Verified via live webhook against a test repo:

1. Open a PR against main; wait for pr_files / pr_file_contents to populate and scoring_data_stored to flip to true.
2. Retarget the PR's base branch (GitHub UI → Edit → change base).
3. Observe in logs: PR_FILES job enqueued for the new (headSha, newBaseSha) generation; scoring_data_stored momentarily false, then true once the new fetch completes.
4. Confirm merge_base_sha is recomputed and pr_file_contents.base_content reflects blobs from the new base branch.
5. Edit only the PR title/body (no base change) and confirm no PR_FILES job is enqueued (only PR_METADATA).

Also ran npm run format:check, npm run lint, and npm run build in packages/das/ — all pass.

Checklist

• I have read the Contributing Guide
• Code builds without errors
• New and existing tests pass (if applicable) — repo has no automated test suite; manual webhook verification used per project convention (commit 76fd0a8)
• Documentation updated (if applicable) — no public API change
• No unnecessary dependencies added