collect recent commit hashes to find nearest ancestor branch

[refoo0]

Closes #2796

Done

• Scanner CLI collects recent commit hashes and sends them via the X-Recent-Commit-Hashes header.
• API stores the hashes in AssetVersion.RecentCommitHashes (jsonb).
• New repository method GetNearestAncestorAssetVersion finds the nearest ancestor AssetVersion via SQL based on shared commits.
• Added migration for the new column.

Open

• GetNearestAncestorAssetVersion isn't wired up yet: scan_service.go still diffs against all other AssetVersions instead of just the real ancestor.
• Fallback behavior when no common commit is found is not yet defined.
• Tests for the new ancestor lookup are still missing.