feat: introduce retention-rate aggregation

packages/backend/src/models/entities/RetentionAggregation.ts

@@ -0,0 +1,35 @@
+import { Entity, PrimaryColumn, Index, Column } from 'typeorm';
+import { id } from '../id.js';
+import type { User } from './User.js';
+
+@Entity()
+export class RetentionAggregation {
+	@PrimaryColumn(id())
+	public id: string;
+
+	@Index()
+	@Column('timestamp with time zone', {
+		comment: 'The created date of the Note.',
+	})
+	public createdAt: Date;
+
+	@Column('timestamp with time zone', {
+		comment: 'The updated date of the GalleryPost.',
+	})
+	public updatedAt: Date;
+
+	@Column({
+		...id(),
+		array: true,
+	})
+	public userIds: User['id'][];
+
+	@Column('integer', {
+	})
+	public usersCount: number;
+
+	@Column('jsonb', {
+		default: {},
+	})
+	public data: Record<string, number>;
+}