[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-object-metadata/validators/utils/validate-flat-object-metadata-identifiers.util.ts

@@ -2,6 +2,7 @@ import { msg } from '@lingui/core/macro';
 import {
   isDefined,
   isLabelIdentifierFieldMetadataTypes,
+  isSearchableFieldType,
 } from 'twenty-shared/utils';
 
 import { findFlatEntityByUniversalIdentifier } from 'src/engine/metadata-modules/flat-entity/utils/find-flat-entity-by-universal-identifier.util';
@@ -50,7 +51,14 @@ export const validateFlatObjectMetadataIdentifiers = ({
         code: ObjectMetadataExceptionCode.INVALID_OBJECT_INPUT,
         message:
           'labelIdentifierFieldMetadataUniversalIdentifier validation failed: field type not compatible',
-        userFriendlyMessage: msg`Field cannot be used as label identifier`,
+        userFriendlyMessage: msg`Field cannot be used as label identifier due to its type: should be of type UUID, text or full name`,
+      });
+    } else if (!isSearchableFieldType(universalFlatFieldMetadata.type)) {
+      errors.push({
+        code: ObjectMetadataExceptionCode.INVALID_OBJECT_INPUT,
+        message:
+          'labelIdentifierFieldMetadataUniversalIdentifier validation failed: field type not compatible',
+        userFriendlyMessage: msg`Field cannot be used as label identifier due to its type`,
       });
     }
   }