Add CERT_DAYS setting and resolve_cert_days()

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_config_file.py

@@ -60,3 +60,10 @@ def test_comments_and_non_string_types_supported(tmp_path):
     )
     overrides = load_settings_overrides(None, "", _script_path(tmp_path))
     assert overrides == {"SMTP_PORT": 2525, "CA_PASSPHRASE": ""}
+
+
+def test_cert_days_is_overridable(tmp_path):
+    conf = tmp_path / "app.conf"
+    conf.write_text('CERT_DAYS = 90\n')
+    out = load_settings_overrides(str(conf), "", str(tmp_path / "app.py"))
+    assert out["CERT_DAYS"] == 90