feat(server): improve calendar sync queue (#14783)

#### PR Dependency Tree


* **PR #14783** 👈

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**
  * Configurable request timeout for calendar integrations.
* Calendar polling now enqueues per-subscription sync jobs (larger
batch) for improved throughput.

* **Bug Fixes / Improvements**
* Persisted next-sync timestamps and retry counts for more reliable
scheduling and retry behavior.
* Exponential backoff and webhook renewal now update scheduling
consistently.

* **Refactor**
* Calendar sync flow moved to a job-queue-driven design for better
concurrency and observability.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/schema.prisma

@@ -1037,6 +1037,8 @@ model CalendarSubscription {
   enabled            Boolean   @default(true)
   syncToken          String?   @map("sync_token") @db.Text
   lastSyncAt         DateTime? @map("last_sync_at") @db.Timestamptz(3)
+  nextSyncAt         DateTime  @default(now()) @map("next_sync_at") @db.Timestamptz(3)
+  syncRetryCount     Int       @default(0) @map("sync_retry_count")
   customChannelId    String?   @map("custom_channel_id") @db.VarChar
   customResourceId   String?   @map("custom_resource_id") @db.VarChar
   channelExpiration  DateTime? @map("channel_expiration") @db.Timestamptz(3)
@@ -1050,6 +1052,8 @@ model CalendarSubscription {
   @@unique([accountId, externalCalendarId])
   @@index([accountId])
   @@index([provider, externalCalendarId])
+  @@index([customChannelId])
+  @@index([enabled, nextSyncAt])
   @@map("calendar_subscriptions")
 }