feat (search): add dynamic search field metadata foundation for database-driven search configuration (#14469)

## Summary

Implements the foundation for dynamic search field configuration from
[issue #1428](https://github.com/twentyhq/core-team-issues/issues/1428).

## Changes

- Add `SearchFieldMetadataEntity` junction table for storing searchable
field configurations
- Add `SearchFieldMetadataService` with core CRUD operations  
- Add `SearchFieldMetadataModule` following existing patterns  
- Add `IS_DYNAMIC_SEARCH_FIELDS_ENABLED` feature flag (defaults to
`false`)
- Database migration with proper indexes and foreign keys  

## Architecture

Uses a junction table where **record existence = field is searchable**:

**Table: `searchFieldMetadata(objectMetadataId, fieldMetadataId,
workspaceId)`**

- Unique constraint on `(objectMetadataId, fieldMetadataId)`  
- `ON DELETE CASCADE` on foreign keys  

## Testing

- [x] Migration runs successfully  
- [x] Table created with correct schema  
- [x] Feature flag seeded properly  
- [x] Database reset works correctly  

## Next Steps

Future PRs will handle:

- Data migration from existing hardcoded search field configs  
- Integration with search services  

✅ No breaking changes — fully backward compatible.

packages/twenty-server/src/engine/core-modules/feature-flag/enums/feature-flag-key.enum.ts

@@ -18,4 +18,5 @@ export enum FeatureFlagKey {
   IS_CALENDAR_VIEW_ENABLED = 'IS_CALENDAR_VIEW_ENABLED',
   IS_GROUP_BY_ENABLED = 'IS_GROUP_BY_ENABLED',
   IS_PUBLIC_DOMAIN_ENABLED = 'IS_PUBLIC_DOMAIN_ENABLED',
+  IS_DYNAMIC_SEARCH_FIELDS_ENABLED = 'IS_DYNAMIC_SEARCH_FIELDS_ENABLED',
 }