feat: CREATE INDEX CONCURRENTLY for "userId" "id" composite note index if admin wish. (#15915)

* chore: CREATE INDEX CONCURRENTLY for "userId" "id" composite note index

* chore: remove { concurrent: true } and comment why

* update comment

* feat: add MISSKEY_MIGRATION_CREATE_INDEX_CONCURRENTLY option

* fix: spdx license header

* alter comment

* chore: improve behavior when migration failure

* docs(changelog): 2025.4.1 で追加されたインデックスの再生成をノートの追加しながら行えるようになりました

* ちょっと表現を変更

---------

Co-authored-by: 饺子w (Yumechi) <35571479+eternal-flame-ad@users.noreply.github.com>

packages/backend/migration/js/migration-config.js

@@ -0,0 +1,8 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+export function isConcurrentIndexMigrationEnabled() {
+	return process.env.MISSKEY_MIGRATION_CREATE_INDEX_CONCURRENTLY === '1';
+}