Fix flat entity maps date serialization (#16420)

Changes:
- as we store date in redis as serialized, let's make all flatEntity
dates as string. This requires changing FlatEntity types and making sure
that entity are converted to flatEntity and flatEntity to dtos

packages/twenty-server/src/engine/metadata-modules/flat-page-layout-widget/utils/from-create-page-layout-widget-input-to-flat-page-layout-widget-to-create.util.ts

@@ -23,7 +23,7 @@ export const fromCreatePageLayoutWidgetInputToFlatPageLayoutWidgetToCreate = ({
       ['pageLayoutTabId'],
     );
 
-  const createdAt = new Date();
+  const createdAt = new Date().toISOString();
   const pageLayoutWidgetId = v4();
 
   return {

packages/twenty-server/src/engine/metadata-modules/flat-page-layout-widget/utils/from-delete-page-layout-widget-input-to-flat-page-layout-widget-or-throw.util.ts

@@ -36,6 +36,6 @@ export const fromDeletePageLayoutWidgetInputToFlatPageLayoutWidgetOrThrow = ({
 
   return {
     ...existingFlatPageLayoutWidgetToDelete,
-    deletedAt: new Date(),
+    deletedAt: new Date().toISOString(),
   };
 };

packages/twenty-server/src/engine/metadata-modules/flat-page-layout-widget/utils/from-page-layout-widget-entity-to-flat-page-layout-widget.util.ts

@@ -14,6 +14,9 @@ export const fromPageLayoutWidgetEntityToFlatPageLayoutWidget = (
 
   return {
     ...pageLayoutWidgetEntityWithoutRelations,
+    createdAt: pageLayoutWidgetEntity.createdAt.toISOString(),
+    updatedAt: pageLayoutWidgetEntity.updatedAt.toISOString(),
+    deletedAt: pageLayoutWidgetEntity.deletedAt?.toISOString() ?? null,
     universalIdentifier:
       pageLayoutWidgetEntityWithoutRelations.universalIdentifier ??
       pageLayoutWidgetEntityWithoutRelations.id,