feat: handle calendar sync failed (#14510)

#### PR Dependency Tree


* **PR #14510** 👈

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

* **Bug Fixes**
* Improved calendar sync reliability with exponential backoff for
repeated failures.
* Better handling of token refresh failures with automatic account
invalidation and cleanup when needed.
* Subscriptions are now automatically disabled and related events
removed when the calendar provider reports missing resources.

* **Tests**
* Added comprehensive tests covering sync failures, backoff behavior,
token refresh handling, skipping retries during backoff, and recovery.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/models/calendar-account.ts

@@ -1,4 +1,5 @@
 import { Injectable } from '@nestjs/common';
+import { Transactional } from '@nestjs-cls/transactional';
 import type { CalendarAccount, Prisma } from '@prisma/client';
 
 import { CryptoHelper } from '../base';
@@ -174,6 +175,18 @@ export class CalendarAccountModel extends BaseModel {
     });
   }
 
+  @Transactional()
+  async invalidateAndPurge(id: string, lastError?: string | null) {
+    await this.updateStatus(id, 'invalid', lastError ?? null);
+    const subscriptions =
+      await this.models.calendarSubscription.listByAccount(id);
+    const subscriptionIds = subscriptions.map(subscription => subscription.id);
+    if (subscriptionIds.length > 0) {
+      await this.models.calendarEvent.deleteBySubscriptionIds(subscriptionIds);
+    }
+    await this.models.calendarSubscription.clearSyncTokensByAccount(id);
+  }
+
   async delete(id: string) {
     return await this.db.calendarAccount.delete({
       where: { id },

packages/backend/server/src/models/calendar-subscription.ts

@@ -1,4 +1,5 @@
 import { Injectable } from '@nestjs/common';
+import { Transactional } from '@nestjs-cls/transactional';
 import type { CalendarSubscription, Prisma } from '@prisma/client';
 
 import { BaseModel } from './base';
@@ -191,4 +192,20 @@ export class CalendarSubscriptionModel extends BaseModel {
       data,
     });
   }
+
+  @Transactional()
+  async disableAndPurge(subscriptionId: string) {
+    await this.db.calendarSubscription.update({
+      where: { id: subscriptionId },
+      data: {
+        enabled: false,
+        syncToken: null,
+        customChannelId: null,
+        customResourceId: null,
+        channelExpiration: null,
+      },
+    });
+
+    await this.models.calendarEvent.deleteBySubscriptionIds([subscriptionId]);
+  }
 }