unique constraint for registry_item - fixes #1121

we remove all duplicate items from the registry, keeping the
most-recently updated one (which presumably is the one that the users
expect to be used)

then we use `upsert` instead of the more complicated previous logic,
which was subject to races

packages/backend/src/models/RegistryItem.ts

@@ -7,8 +7,8 @@ import { PrimaryColumn, Entity, Index, JoinColumn, Column, ManyToOne } from 'typ
 import { id } from './util/id.js';
 import { MiUser } from './User.js';
 
-// TODO: 同じdomain、同じscope、同じkeyのレコードは二つ以上存在しないように制約付けたい
 @Entity('registry_item')
+@Index(['userId', 'key', 'scope', 'domain'], { unique: true })
 export class MiRegistryItem {
 	@PrimaryColumn(id())
 	public id: string;