[enh] checker: background check

See settings.yml for the options
SIGUSR1 signal starts the checker.
The result is available at /stats/checker

searx/search/checker/__init__.py

@@ -1 +1,4 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
 from .impl import Checker
+from .background import initialize, get_result