chore: update CHANGELOG for #1063

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: brendan-kellam

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Added `GET /api/diff` endpoint for retrieving structured diffs between two git refs ([#1063](https://github.com/sourcebot-dev/sourcebot/pull/1063))
+
 ## [4.16.3] - 2026-03-27
 
 ### Added