Give a library book one form in the API

A book from the search, from the list of library candidates and from a
lookup now has the same fields, with the authors as a list.

tests/test_api_flows.py

@@ -91,7 +91,7 @@ def test_library_search_link_import_and_unlink(client, story, monkeypatch):
     cwa.get_book.return_value = book.to_dict()
     monkeypatch.setattr(o, "cwa", lambda: cwa)
     monkeypatch.setattr(config, "CWA_URL", "https://cwa.example")
-    assert client.get("/api/library/candidates").json[0]["calibre_id"] == 7
+    assert client.get("/api/library/candidates").json[0]["id"] == 7
     assert client.get("/api/library/search?q=Title").json[0]["tracked_as"] is None
     cwa.search.assert_called_once_with("Title")
     linked = client.post(f"/api/stories/{sid}/link", json={"uuid": "book-7"}, headers=H)