feat(backend): add /healthz endpoint (#13834)

* feat(backend): add /healthz endpoint

* feat(backend): also check meilisearch status if available

* style: header

* chore: no-store

* chore: healthcheck.sh

* style: format

packages/backend/src/boot/ready.ts

@@ -0,0 +1,6 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+export const readyRef = { value: false };