[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/workspace-manager/workspace-migration/universal-flat-entity/types/__tests__/universal-flat-entity-update.test-type.ts

@@ -35,6 +35,7 @@ type Assertions = [
       | 'labelSingular'
       | 'labelPlural'
       | 'labelIdentifierFieldMetadataUniversalIdentifier'
+      | 'isSearchable'
     >
   >,
 ];