[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-shared/src/application/objectManifestType.ts

@@ -8,6 +8,7 @@ export type ObjectManifest = SyncableEntityOptions & {
   labelPlural: string;
   description?: string;
   icon?: string;
+  isSearchable?: boolean;
   fields: ObjectFieldManifest[];
   labelIdentifierFieldMetadataUniversalIdentifier: string;
 };