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/frontend/admin/src/config.json

@@ -27,6 +27,10 @@
       "type": "Object",
       "desc": "The config for copilot job queue"
     },
+    "queues.calendar": {
+      "type": "Object",
+      "desc": "The config for calendar job queue"
+    },
     "queues.doc": {
       "type": "Object",
       "desc": "The config for doc job queue"