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/admin.py

@@ -61,50 +61,20 @@ async def scan_library(request: Request, config: AppConfig, session: AsyncDbSess
 
 
 @router.post("/phrases/generate-all", response_class=HTMLResponse)
-async def generate_all_phrases(request: Request, config: AppConfig, session: AsyncDbSession) -> HTMLResponse:
+async def generate_all_phrases(request: Request, config: AppConfig, engine: AppEngine) -> HTMLResponse:
     """Regenerate candidate phrases for every indexed book without LLM judging."""
-    return await run_phrase_generation(request, config, session)
-
-
-async def run_phrase_generation(
-    request: Request,
-    config: AppConfig,
-    session: AsyncDbSession,
-    *,
-    only_missing: bool,
-) -> HTMLResponse:
-    """Run candidate phrase generation and render the outcome as an admin status partial.
-
-    Args:
-        request (Request): Current request, for template rendering.
-        config (AppConfig): Runtime phrase-tuning settings.
-        session (AsyncDbSession): Active database session.
-        only_missing (bool): Only generate for books without candidates instead of every book.
-
-    Returns:
-        HTMLResponse: Status partial describing the generation outcome.
-    """
     try:
-        result = await generate_candidate_phrases_for_books(session, config, only_missing=only_missing)
-        await session.commit()
+        result = await generate_candidate_phrases_for_books(engine, config)
     except Exception as error:
-        await session.rollback()
-        logger.exception("ebook_admin_generate_phrases_failed only_missing=%s", only_missing)
+        logger.exception("ebook_admin_generate_phrases_failed")
         return templates.TemplateResponse(request, "partials/error.html", {"message": str(error)}, status_code=500)
 
     logger.info(
-        "ebook_admin_generate_phrases_complete only_missing=%s books_seen=%s books_built=%s candidates=%s",
-        only_missing,
+        "ebook_admin_generate_phrases_complete books_seen=%s books_built=%s candidates=%s",
         result.books_seen,
         result.books_built,
         result.candidate_phrases,
     )
-    if only_missing and result.books_seen == 0:
-        return templates.TemplateResponse(
-            request,
-            "partials/admin_status.html",
-            {"message": "All books already have candidate phrases"},
-        )
     return templates.TemplateResponse(
         request,
         "partials/admin_status.html",