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.

tests/ebook_search/test_ui.py

@@ -567,33 +567,8 @@ def test_admin_page_shows_protected_phrase_stats(mocker: MockerFixture) -> None:
         assert value in response.text
 
 
-def test_ui_add_missing_phrases_generates_only_missing_books(mocker: MockerFixture) -> None:
-    captured: dict[str, object] = {}
-
-    def fake_generate(_session, _config, *, only_missing=False):
-        captured["only_missing"] = only_missing
-        return PhraseCandidateGenerationResult(books_seen=3, books_built=2, candidate_phrases=42)
-
-    mocker.patch(
-        "python.ebook_search.api.routes.admin.generate_candidate_phrases_for_books",
-        side_effect=fake_generate,
-    )
-    patch_app_runtime(mocker)
-    app = create_app()
-
-    with TestClient(app) as client:
-        response = client.post("/admin/phrases/generate-missing")
-
-    assert response.status_code == 200
-    assert captured["only_missing"] is True
-    assert "42 candidates stored" in response.text
-
-
 def test_ui_regenerate_all_phrases_generates_every_book(mocker: MockerFixture) -> None:
-    captured: dict[str, object] = {}
-
-    def fake_generate(_session, _config, *, only_missing=False):
-        captured["only_missing"] = only_missing
+    def fake_generate(_session, _config):
         return PhraseCandidateGenerationResult(books_seen=5, books_built=5, candidate_phrases=99)
 
     mocker.patch(
@@ -607,7 +582,6 @@ def test_ui_regenerate_all_phrases_generates_every_book(mocker: MockerFixture) -
         response = client.post("/admin/phrases/generate-all")
 
     assert response.status_code == 200
-    assert captured["only_missing"] is False
     assert "5 of 5 books" in response.text