feat: basic caldav support (#14372)

fix #13531 

#### PR Dependency Tree


* **PR #14372** 👈

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**
* CalDAV calendar integration: link and sync CalDAV-compatible calendars
(discovery, listing, event sync).
* New UI flow and dialog to link CalDAV accounts with provider
selection, credentials, and display name.

* **API / Config**
* Server exposes CalDAV provider presets in config and new GraphQL
mutation to link CalDAV accounts.
  * New calendar config section for CalDAV with validation and defaults.

* **Tests**
  * Comprehensive CalDAV integration test suite added.

* **Chores**
* Removed analytics tokens from build configuration and reduced Cloud
E2E test shards.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/migrations/20260110095744_caldav_fields/migration.sql

@@ -0,0 +1,7 @@
+-- AlterTable
+ALTER TABLE "calendar_accounts" ADD COLUMN     "auth_type" VARCHAR,
+ADD COLUMN     "calendar_home_url" VARCHAR,
+ADD COLUMN     "principal_url" VARCHAR,
+ADD COLUMN     "provider_preset_id" VARCHAR,
+ADD COLUMN     "server_url" VARCHAR,
+ADD COLUMN     "username" VARCHAR;