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

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

packages/backend/src/models/Note.ts

@@ -10,6 +10,16 @@ import { MiUser } from './User.js';
 import { MiChannel } from './Channel.js';
 import type { MiDriveFile } from './DriveFile.js';
 
+// Note: When you create a new index for existing column of this table,
+// it might be better to index concurrently under isConcurrentIndexMigrationEnabled flag
+// by editing generated migration file since this table is very large,
+// and it will make a long lock to create index in most cases.
+// Please note that `CREATE INDEX CONCURRENTLY` is not supported in transaction,
+// so you need to set `transaction = false` in migration if isConcurrentIndexMigrationEnabled() is true.
+// Please refer 1745378064470-composite-note-index.js for example.
+// You should not use `@Index({ concurrent: true })` decorator because database initialization for test will fail
+// because it will always run CREATE INDEX in transaction based on decorators.
+// Not appending `{ concurrent: true }` to `@Index` will not cause any problem in production,
 @Index(['userId', 'id'])
 @Entity('note')
 export class MiNote {