fix(backend): バックエンドのpnpm devによるビルド後にbuild-assetsを行うようにする (#13659)

* moveto scripts

* add scripts/dev.mjs

packages/backend/scripts/generate_api_json.js

@@ -0,0 +1,13 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { loadConfig } from '../built/config.js'
+import { genOpenapiSpec } from '../built/server/api/openapi/gen-spec.js'
+import { writeFileSync } from "node:fs";
+
+const config = loadConfig();
+const spec = genOpenapiSpec(config, true);
+
+writeFileSync('./built/api.json', JSON.stringify(spec), 'utf-8');