feat: init cloud calendar support (#14247)

#### PR Dependency Tree


* **PR #14247** 👈
  * **PR #14248**

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**
* Google Calendar integration (disabled by default): link/unlink
accounts, OAuth flow, webhooks, real-time push, background sync,
workspace calendars with customizable items and date-range event
viewing.
* **GraphQL / Client**
* New queries & mutations for accounts, subscriptions, events,
providers, and workspace calendar management.
* **Localization**
* Added localized error message for calendar provider request failures.
* **Tests**
* Backend tests covering sync, webhook renewal, and error/error-recovery
scenarios.

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settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/models/calendar-event-instance.ts

@@ -0,0 +1,16 @@
+import { Injectable } from '@nestjs/common';
+
+import { BaseModel } from './base';
+
+@Injectable()
+export class CalendarEventInstanceModel extends BaseModel {
+  async deleteByEventIds(eventIds: string[]) {
+    if (eventIds.length === 0) {
+      return;
+    }
+
+    await this.db.calendarEventInstance.deleteMany({
+      where: { calendarEventId: { in: eventIds } },
+    });
+  }
+}