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/schema.prisma

@@ -937,6 +937,12 @@ model CalendarAccount {
   providerAccountId      String    @map("provider_account_id") @db.VarChar
   displayName            String?   @map("display_name") @db.VarChar
   email                  String?   @db.VarChar
+  providerPresetId       String?   @map("provider_preset_id") @db.VarChar
+  serverUrl              String?   @map("server_url") @db.VarChar
+  principalUrl           String?   @map("principal_url") @db.VarChar
+  calendarHomeUrl        String?   @map("calendar_home_url") @db.VarChar
+  username               String?   @db.VarChar
+  authType               String?   @map("auth_type") @db.VarChar
   accessToken            String?   @map("access_token") @db.Text
   refreshToken           String?   @map("refresh_token") @db.Text
   expiresAt              DateTime? @map("expires_at") @db.Timestamptz(3)