Check every setting before the first one is stored

A bad value in a settings request no longer leaves the other values of
that request saved.

tests/test_api.py

@@ -280,6 +280,33 @@ def test_delete_revisions(client):
     assert [r["rev"] for r in client.get(f"/api/chapters/{chapter_id}/diff").json["revisions"]] == [1, 3]
 
 
+@pytest.mark.parametrize(
+    "bad",
+    [
+        {"recent_chapters": "many"},
+        {"main_jobs_count": 100000},
+        {"personal_ini": "[defaults\nfoo"},
+        {"link_books": "yes"},
+    ],
+)
+def test_a_bad_setting_changes_no_setting(client, monkeypatch, tmp_path, bad):
+    ini = tmp_path / "personal.ini"
+    ini.write_text("[defaults]\n")
+    monkeypatch.setattr(config, "PERSONAL_INI", ini)
+    monkeypatch.setattr(config, "CWA_URL", "https://cwa.invalid")
+    before = client.get("/api/settings").json
+    good = {
+        "link_books": False,
+        "hold_new_links": True,
+        "pause_uploads": True,
+        "recent_chapters": 4,
+        "story_jobs_hours": 1,
+        "personal_ini": "[defaults]\nis_adult:true\n",
+    }
+    assert client.put("/api/settings", json={**good, **bad}, headers=H).status_code == 400
+    assert client.get("/api/settings").json == before
+
+
 def test_settings(client, monkeypatch, tmp_path):
     ini = tmp_path / "personal.ini"
     ini.write_text("[defaults]\nis_adult:true\n")