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-entity/constant/create-empty-flat-entity-maps.constant.ts

@@ -1,9 +1,9 @@
 import { type FlatEntityMaps } from 'src/engine/metadata-modules/flat-entity/types/flat-entity-maps.type';
-import { type FlatEntity } from 'src/engine/metadata-modules/flat-entity/types/flat-entity.type';
+import { type SyncableFlatEntity } from 'src/engine/metadata-modules/flat-entity/types/flat-entity.type';
 
 export const createEmptyFlatEntityMaps = () =>
   ({
     byId: {},
     idByUniversalIdentifier: {},
     universalIdentifiersByApplicationId: {},
-  }) as const satisfies FlatEntityMaps<FlatEntity>;
+  }) as const satisfies FlatEntityMaps<SyncableFlatEntity>;