Share the checks and the job builders of the orchestrator

One function checks a whole number of a request, and one checks the
metadata field names. save_settings takes the settings by name. A rebuild
job and a story from an epub each have one builder.

tests/test_api_flows.py

@@ -199,3 +199,17 @@ def test_a_story_change_does_not_ask_cwa_for_the_book(client, story, monkeypatch
     assert response.json["recheck_days"] == 4
     assert "chapters" not in response.json
     cwa.get_book.assert_not_called()
+
+
+def test_settings_are_saved_by_name(client):
+    with pytest.raises(ValueError, match="Unknown settings: \\['recent'\\]"):
+        o.save_settings(recent=3)
+    # The route passes only the known settings on.
+    response = client.put("/api/settings", json={"recent": 3, "recent_chapters": 4, "main_jobs_count": 9}, headers=H)
+    assert (response.status_code, response.json["recent_chapters"], response.json["main_jobs_count"]) == (200, 4, 9)
+    for data, error in [
+        ({"recent_chapters": -1}, "The number of recent chapters cannot be negative"),
+        ({"main_jobs_count": 501}, "main_jobs_count cannot be more than 500"),
+        ({"story_jobs_hours": "x"}, "story_jobs_hours must be a whole number"),
+    ]:
+        assert client.put("/api/settings", json=data, headers=H).json == {"error": error}