fix(protected-phrases): isolate phrase generation per book

Run full-book candidate generation inside worker-owned sessions so each book commits independently during backfills. Abort recalculation when a book has no indexed chapters to preserve existing phrase data, and update admin/UI tests for the new generation flow.

python/ebook_search/api/routes/page.py

@@ -168,7 +168,10 @@ async def recalculate_book_phrases(source_id: int, config: AppConfig, session: A
     if source is None:
         raise HTTPException(status_code=404, detail="Book not found")
 
-    result = await recalculate_candidate_phrases_for_book(session, source, config, use_process_pool=True)
+    try:
+        result = await recalculate_candidate_phrases_for_book(session, source, config)
+    except ValueError as error:
+        raise HTTPException(status_code=409, detail=str(error)) from error
     logger.info(
         "ebook_book_phrase_recalculation_complete source_id=%s candidates=%s deleted_candidates=%s "
         "deleted_protected=%s deleted_aliases=%s deleted_mentions=%s",