feat: gate record creation on isUICreatable only, decoupled from isSystem (#21527)

## Context

The generic "create a record" UI affordance previously required
`!isSystem`, conflating two distinct concerns: **"hidden from Data
Model"** and **"not user-creatable"**. This blocked legitimately
creatable system objects (e.g. marketing message lists kept `isSystem:
true` only to stay out of the Data Model).

This PR makes creatability depend on `isUICreatable` alone, so
visibility (`isSystem`) and creatability (`isUICreatable`) become
independent.

## Changes

- **Front gate** (`canCreateRecordsForObjectMetadataItem.ts`): drop the
`!isSystem` clause — creatability is now `isUICreatable && !readOnly`.
Updated comment + unit test.
- **Command menu** (`standard-command-menu-item.constant.ts`): drop the
matching `not objectMetadataItem.isSystem` clause from the
`createNewRecord` availability expression so the "Create new X" command
mirrors the front gate. Existing workspaces get this via the
already-present `SyncCreateRecordCommandAvailabilityExpressionCommand`,
which re-syncs from the live definition.
- **Standard object audit**
(`create-standard-flat-object-metadata.util.ts`): the 15
sync/system-created standard objects that relied on `!isSystem` to stay
non-creatable now set `isUICreatable: false` (attachment, blocklist,
calendar*/message*/note/task targets, message, messageThread,
messageParticipant, timelineActivity, callRecording,
workflowAutomatedTrigger, …).
`workflowRun`/`workflowVersion`/`workspaceMember` were already `false`.
Non-system objects (company, person, note, opportunity, dashboard, task,
workflow) are untouched.
- **Backfill**: new fast instance command
(`2-13-…-1781277480000-backfill-non-ui-creatable-standard-system-objects.ts`)
runs `UPDATE core.objectMetadata SET isUICreatable = false` for those
standard system objects (symmetric `down`), registered in
`instance-commands.constant.ts`.

`isSystem` and Data-Model visibility logic are unchanged.

## Verification

- Front gate unit test (7 pass), standard-application suite incl.
callRecording (10 pass)
- `oxlint` clean on all changed files
- `twenty-server` and `twenty-front` typecheck green

🤖 https://claude.ai/code/session_01TF4kjD56hHjP31wkHPxPv3

---
_Generated by [Claude
Code](https://claude.ai/code/session_01TF4kjD56hHjP31wkHPxPv3)_

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---------

Co-authored-by: Claude <noreply@anthropic.com>

packages/twenty-server/src/database/commands/upgrade-version-command/instance-commands.constant.ts

@@ -63,6 +63,7 @@ import { DropIsCustomFromObjectAndFieldMetadataFastInstanceCommand } from 'src/d
 import { DropEmailingDomainDriverColumnFastInstanceCommand } from 'src/database/commands/upgrade-version-command/2-11/2-11-instance-command-fast-1780926908000-drop-emailing-domain-driver-column';
 import { ViewOverridableEntityFastInstanceCommand } from 'src/database/commands/upgrade-version-command/2-12/2-12-instance-command-fast-1781114009075-view-overridable-entity';
 import { RenameIsUiReadOnlyToIsUiEditableFastInstanceCommand } from 'src/database/commands/upgrade-version-command/2-13/2-13-instance-command-fast-1781277453604-rename-is-ui-read-only-to-is-ui-editable';
+import { BackfillNonUiCreatableStandardSystemObjectsSlowInstanceCommand } from 'src/database/commands/upgrade-version-command/2-13/2-13-instance-command-slow-1781277480000-backfill-non-ui-creatable-standard-system-objects';
 
 export const INSTANCE_COMMANDS = [
   AddViewFieldGroupIdIndexOnViewFieldFastInstanceCommand,
@@ -128,4 +129,5 @@ export const INSTANCE_COMMANDS = [
   DropEmailingDomainDriverColumnFastInstanceCommand,
   ViewOverridableEntityFastInstanceCommand,
   RenameIsUiReadOnlyToIsUiEditableFastInstanceCommand,
+  BackfillNonUiCreatableStandardSystemObjectsSlowInstanceCommand,
 ];