enhance(backend): refine system account (#15530)

* wip

* wip

* wip

* Update SystemAccountService.ts

* Update 1740121393164-system-accounts.js

* Update DeleteAccountService.ts

* wip

* wip

* wip

* wip

* Update 1740121393164-system-accounts.js

* Update RepositoryModule.ts

* wip

* wip

* wip

* Update ApRendererService.ts

* wip

* wip

* Update SystemAccountService.ts

* fix tests

* fix tests

* fix tests

* fix tests

* fix tests

* fix tests

* add print logs

* ログが長すぎて出てないかもしれない

* fix migration

* refactor

* fix fed-tests

* Update RelayService.ts

* merge

* Update user.test.ts

* chore: emit log

* fix: tweak sleep duration

* fix: exit 1

* fix: wait for misskey processes to become healthy

* fix: longer sleep for user deletion

* fix: make sleep longer again

* デッドロック解消の試み

https://github.com/misskey-dev/misskey/issues/15005

* Revert "デッドロック解消の試み"

This reverts commit 266141f66f.

* wip

* Update SystemAccountService.ts

---------

Co-authored-by: おさむのひと <46447427+samunohito@users.noreply.github.com>
Co-authored-by: zyoshoka <107108195+zyoshoka@users.noreply.github.com>

packages/backend/src/models/SystemAccount.ts

@@ -0,0 +1,31 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { Column, Entity, Index, JoinColumn, ManyToOne, PrimaryColumn } from 'typeorm';
+import { Serialized } from '@/types.js';
+import { id } from './util/id.js';
+import { MiUser } from './User.js';
+
+@Entity('system_account')
+@Index(['type'], { unique: true })
+export class MiSystemAccount {
+	@PrimaryColumn(id())
+	public id: string;
+
+	@Index()
+	@Column(id())
+	public userId: MiUser['id'];
+
+	@ManyToOne(type => MiUser, {
+		onDelete: 'CASCADE',
+	})
+	@JoinColumn()
+	public user: MiUser | null;
+
+	@Column('varchar', {
+		length: 256,
+	})
+	public type: string;
+}