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-front/src/modules/settings/data-model/objects/components/SettingsDataModelObjectSummary.tsx

@@ -16,7 +16,7 @@ export type SettingsDataModelObjectPreviewProps = {
     | 'icon'
     | 'labelSingular'
     | 'labelPlural'
-    | 'isCustom'
+    | 'applicationId'
     | 'isRemote'
     | 'nameSingular'
     | 'color'
@@ -64,7 +64,7 @@ type SettingsDataModelObjectPreviewItemProps = {
     | 'icon'
     | 'labelSingular'
     | 'labelPlural'
-    | 'isCustom'
+    | 'applicationId'
     | 'isRemote'
     | 'nameSingular'
     | 'color'

packages/twenty-front/src/modules/settings/data-model/objects/forms/components/SettingsDataModelObjectAboutForm.tsx

@@ -1,3 +1,4 @@
+import { useGetIsMetadataItemCustom } from '@/object-metadata/hooks/useGetIsMetadataItemCustom';
 import { type EnrichedObjectMetadataItem } from '@/object-metadata/types/EnrichedObjectMetadataItem';
 import { AdvancedSettingsWrapper } from '@/settings/components/AdvancedSettingsWrapper';
 import { SettingsOptionCardContentToggle } from '@/settings/components/SettingsOptions/SettingsOptionCardContentToggle';
@@ -80,21 +81,25 @@ export const SettingsDataModelObjectAboutForm = ({
     useFormContext<SettingsDataModelObjectAboutFormValues>();
   const { t } = useLingui();
   const navigateSettings = useNavigateSettings();
+  const getIsMetadataItemCustom = useGetIsMetadataItemCustom();
 
   const isLabelSyncedWithName = watch('isLabelSyncedWithName');
   const labelSingular = watch('labelSingular');
   const labelPlural = watch('labelPlural');
   const isStandardObject =
-    isDefined(objectMetadataItem?.isCustom) && !objectMetadataItem.isCustom;
+    isDefined(objectMetadataItem) &&
+    !getIsMetadataItemCustom(objectMetadataItem);
   const showObjectColorInIconPicker =
     !isStandardObject &&
-    (!isDefined(objectMetadataItem) || objectMetadataItem.isCustom);
+    (!isDefined(objectMetadataItem) ||
+      getIsMetadataItemCustom(objectMetadataItem));
   watch('description');
   watch('icon');
   const objectIconColor = watch('color');
 
   const apiNameTooltipText =
-    !isDefined(objectMetadataItem) || objectMetadataItem.isCustom
+    !isDefined(objectMetadataItem) ||
+    getIsMetadataItemCustom(objectMetadataItem)
       ? isLabelSyncedWithName
         ? t`Deactivate "Synchronize Objects Labels and API Names" to set a custom API name`
         : t`Input must be in camel case and cannot start with a number`
@@ -370,7 +375,7 @@ export const SettingsDataModelObjectAboutForm = ({
                           onChange(value);
                           const isCustomObject =
                             isDefined(objectMetadataItem) &&
-                            objectMetadataItem.isCustom;
+                            getIsMetadataItemCustom(objectMetadataItem);
                           const isbeingCreatedObject =
                             !isDefined(objectMetadataItem);
                           if (

packages/twenty-front/src/modules/settings/data-model/objects/forms/components/SettingsDataModelObjectIdentifiersForm.tsx

@@ -1,4 +1,5 @@
 import { isDDLLockedState } from '@/client-config/states/isDDLLockedState';
+import { useGetIsMetadataItemCustom } from '@/object-metadata/hooks/useGetIsMetadataItemCustom';
 import { useUpdateOneObjectMetadataItem } from '@/object-metadata/hooks/useUpdateOneObjectMetadataItem';
 import { type EnrichedObjectMetadataItem } from '@/object-metadata/types/EnrichedObjectMetadataItem';
 import { getActiveFieldMetadataItems } from '@/object-metadata/utils/getActiveFieldMetadataItems';
@@ -50,6 +51,9 @@ export const SettingsDataModelObjectIdentifiersForm = ({
 }: SettingsDataModelObjectIdentifiersFormProps) => {
   const isDDLLocked = useAtomStateValue(isDDLLockedState);
 
+  const getIsMetadataItemCustom = useGetIsMetadataItemCustom();
+  const isCustomObject = getIsMetadataItemCustom(objectMetadataItem);
+
   const readonly =
     isObjectMetadataReadOnly({
       objectMetadataItem,
@@ -130,7 +134,7 @@ export const SettingsDataModelObjectIdentifiersForm = ({
               options={options}
               value={value}
               withSearchInput={label === t`Record label`}
-              disabled={!objectMetadataItem.isCustom || readonly}
+              disabled={!isCustomObject || readonly}
               callToActionButton={
                 label === t`Record label`
                   ? {