feat(ebook-search): add load-test CLI for the search service

Add a Typer CLI script that drives POST /search on a running server at a
configurable concurrency and reports latency percentiles (p50/p90/p95/p99),
throughput, and HTTP status distribution. Queries are drawn from the shared
eval JSONL set so load testing and evaluation exercise the same questions.

python/ebook_search/api/routes/search.py

@@ -96,6 +96,8 @@ def search(
         timings=(*response.timings, runtime_step_from_start(answer_step_name, answer_start)),
     )
 
+    for step in response.timings:
+        logger.info("ebook_search_timing step=%r runtime_ms=%.1f", step.name, step.duration_ms)
     logger.info(
         "ebook_search_request_complete results=%s rank_label=%s runtime_ms=%.1f",
         len(response.results),