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/app.module.ts

@@ -50,6 +50,7 @@ import { VersionModule } from './core/version';
 import { WorkspaceModule } from './core/workspaces';
 import { Env } from './env';
 import { ModelsModule } from './models';
+import { CalendarModule } from './plugins/calendar';
 import { CaptchaModule } from './plugins/captcha';
 import { CopilotModule } from './plugins/copilot';
 import { CustomerIoModule } from './plugins/customerio';
@@ -188,6 +189,7 @@ export function buildAppModule(env: Env) {
       CopilotModule,
       CaptchaModule,
       OAuthModule,
+      CalendarModule,
       CustomerIoModule,
       CommentModule,
       AccessTokenModule,