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-field-metadata/utils/generate-index-for-flat-field-metadata.util.ts

@@ -18,7 +18,7 @@ export const generateIndexForFlatFieldMetadata = ({
   workspaceId,
 }: GenerateIndexForFlatFieldMetadataArgs): FlatIndexMetadata => {
   const indexId = v4();
-  const createdAt = new Date();
+  const createdAt = new Date().toISOString();
 
   const flatIndex: FlatIndexMetadata = generateFlatIndexMetadataWithNameOrThrow(
     {

packages/twenty-server/src/engine/metadata-modules/flat-field-metadata/utils/recompute-view-groups-on-enum-flat-field-metadata-is-nullable-update.util.ts

@@ -65,7 +65,7 @@ export const recomputeViewGroupsOnEnumFlatFieldMetadataIsNullableUpdate = ({
     ) {
       const highestViewGroupPosition = highestViewGroupPositionByViewId[viewId];
       const viewGroupId = v4();
-      const createdAt = new Date();
+      const createdAt = new Date().toISOString();
 
       sideEffectResult.flatViewGroupsToCreate.push({
         id: viewGroupId,

packages/twenty-server/src/engine/metadata-modules/flat-field-metadata/utils/recompute-view-groups-on-flat-field-metadata-options-update.util.ts

@@ -89,7 +89,7 @@ export const recomputeViewGroupsOnFlatFieldMetadataOptionsUpdate = ({
 
   const viewIds = Object.keys(viewGroupsByViewId.flatViewGroupRecordByViewId);
 
-  const createdAt = new Date();
+  const createdAt = new Date().toISOString();
   const flatViewGroupsToCreate = createdFieldMetadataOptions.flatMap(
     (option, createdOptionIndex) =>
       viewIds.map<FlatViewGroup>((viewId) => {