docs: add free API key auto-detection fix to changelog

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: shirgoldbird

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- Free API keys (ending with `:fx`) now auto-detect the correct endpoint (`api-free.deepl.com`), matching the behavior of official DeepL SDKs
+
 ### Security
 - Updated `minimatch` from `^9.0.5` to `^10.2.1` to fix ReDoS vulnerability (GHSA-3ppc-4f35-3m26)