Update CHANGELOG for v1.5.1 release

Author: alex-omophub

CHANGELOG.md

@@ -5,6 +5,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.5.1] - 2026-04-08
+
+### Fixed
+
+- **Rate-limit handling**: HTTP client now respects the `Retry-After` header on `429 Too Many Requests` responses and applies exponential backoff with jitter on retries. Previous versions retried only on `502/503/504` with a fixed `2^attempt * 0.5s` schedule and did not back off on `429` at all, so a client that hit the server's rate limit at high volume could burn through thousands of failed requests in a tight loop. The new behavior:
+- Updated `examples/search_concepts.py` to reflect current API.
+
 ## [1.5.0] - 2026-03-26
 
 ### Added