fix(metadata): remove deprecated isCustom from Objects and Fields (#21799)

## Context

Follow-up to #21228, which deprecated `isCustom` on object/field
metadata but kept it exposed because the frontend still relied on it.
This removes it from the GraphQL API and the frontend entirely.

## Implementation

### Server
- Remove `isCustom` `@Field` from the `Object`, `Field`, and
`MinimalObjectMetadata` GraphQL types
- Remove the `isCustom` `@ResolveField` resolvers and the
`isCustomLoader` dataloader (+ payload/interface)
- Remove `isCustom` as an internal `@HideField()` on the Object/Field
DTOs used by the i18n standard-override gate > Use an explicit
isStandard instead (which is the correct gating)

### Frontend
- Add `getIsMetadataItemCustom` helper + `useGetIsMetadataItemCustom`
hook: an item is custom when `applicationId ===
currentWorkspace.workspaceCustomApplication.id`
- Migrate all consumers off `objectMetadataItem.isCustom` /
`fieldMetadataItem.isCustom`; `isRecordFieldReadOnly` now takes a
precomputed `isFieldCustom`
- Drop `isCustom` from the metadata fragment/mutations/minimal query, FE
types, zod schemas, and mock generators; regenerate GraphQL types

## Notes
- Breaking change on the (already-deprecated) `Object.isCustom` /
`Field.isCustom` GraphQL fields and the `isCustom` filter
- FE semantic is "belongs to the workspace custom app" (third-party-app
objects/fields are treated as non-custom)
- `isCustom` on IndexMetadata / View / Skill / Agent is a separate
column and is untouched
- Breaking changes on REST metadata API

packages/twenty-server/src/database/commands/upgrade-version-command/2-9/2-9-workspace-command-1799000035000-add-workflow-run-step-logs-field.command.ts

@@ -87,7 +87,6 @@ export class AddWorkflowRunStepLogsFieldCommand extends ActiveOrSuspendedWorkspa
       icon: 'IconTerminal2',
       isNullable: true,
       isUIReadOnly: true,
-      isCustom: false,
       isSystem: true,
       isActive: true,
       universalIdentifier: STEP_LOGS_FIELD_UNIVERSAL_IDENTIFIER,