[Apps] Fix - app-synced object should be searchable (#19206)

## Summary

- **Make app-synced objects searchable**: `isSearchable` was hardcoded
to `false` and the `searchVector` field was missing the `GENERATED
ALWAYS AS (...)` expression, causing all records to have a `NULL` search
vector and be excluded from search results. Fixed by defaulting
`isSearchable` to `true` (configurable via the object manifest),
computing the `asExpression` from the label identifier field, and
allowing the update-field-action-handler to handle the `null` → defined
`asExpression` transition.
- **Make `isSearchable` updatable on an object**: The property had
`toCompare: false` in the entity properties configuration, so updates
via the API were silently ignored and never persisted. Fixed by setting
`toCompare: true`.

packages/twenty-server/src/engine/metadata-modules/flat-field-metadata/utils/handle-label-identifier-changes-during-field-update.util.ts

@@ -1,5 +1,5 @@
 import { type FieldMetadataType, type FromTo } from 'twenty-shared/types';
-import { findOrThrow } from 'twenty-shared/utils';
+import { findOrThrow, type SearchableFieldType } from 'twenty-shared/utils';
 
 import {
   FieldMetadataException,
@@ -11,7 +11,6 @@ import { type FlatFieldMetadata } from 'src/engine/metadata-modules/flat-field-m
 import { type FlatObjectMetadata } from 'src/engine/metadata-modules/flat-object-metadata/types/flat-object-metadata.type';
 import { SEARCH_VECTOR_FIELD } from 'src/engine/metadata-modules/search-field-metadata/constants/search-vector-field.constants';
 import { getTsVectorColumnExpressionFromFields } from 'src/engine/workspace-manager/utils/get-ts-vector-column-expression.util';
-import { type SearchableFieldType } from 'src/engine/workspace-manager/utils/is-searchable-field.util';
 
 type HandleLabelIdentifierChangesDuringFieldUpdateArgs = {
   flatObjectMetadata: FlatObjectMetadata;