feat(server): realtime notification & task status (#14934)

#### PR Dependency Tree


* **PR #14934** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Full realtime platform added: live notifications, comments, embedding
progress, and transcription task updates via realtime subscriptions.

* **Chores**
* Frontend switched from polling/GraphQL queries to realtime channels;
legacy query fields marked deprecated and client libs updated to use
realtime APIs.

[![Review Change
Stack](https://storage.googleapis.com/coderabbit_public_assets/review-stack-in-coderabbit-ui.svg)](https://app.coderabbit.ai/change-stack/toeverything/AFFiNE/pull/14934)
<!-- end of auto-generated comment: release notes by coderabbit.ai -->


#### PR Dependency Tree


* **PR #14934** 👈
  * **PR #14936**

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

packages/backend/server/src/models/notification.ts

@@ -311,6 +311,15 @@ export class NotificationModel extends BaseModel {
     return row as UnionNotification;
   }
 
+  async findExpiredNotificationUserIds() {
+    const rows = await this.db.notification.findMany({
+      distinct: ['userId'],
+      select: { userId: true },
+      where: { createdAt: { lte: new Date(Date.now() - ONE_YEAR) } },
+    });
+    return rows.map(row => row.userId);
+  }
+
   async cleanExpiredNotifications() {
     const { count } = await this.db.notification.deleteMany({
       // delete notifications that are older than one year