Remove DataSourceService and clean up datasource migration logic (#19532)

## Summary

- **Drop the `objectMetadata.dataSourceId` foreign key and index** via a
1-22 fast instance command — column kept nullable for data preservation
- **Delete `DataSourceService`, `DataSourceModule`, and
`DataSourceException`** — all code now uses `workspace.databaseSchema`
directly
- **Remove `IS_DATASOURCE_MIGRATED` feature flag** from default flags
and all branching logic
- **Simplify workspace/object creation pipelines** —
`WorkspaceManagerService`, `DevSeederService`, and the object creation
action handler no longer route through `DataSourceService`
- **Keep `DataSourceEntity` and the `dataSource` table** for historical
data — entity stripped of all ORM relations

packages/twenty-server/src/engine/twenty-orm/twenty-orm.module.ts

@@ -4,7 +4,6 @@ import { TypeOrmModule } from '@nestjs/typeorm';
 import { FeatureFlagModule } from 'src/engine/core-modules/feature-flag/feature-flag.module';
 import { TwentyConfigModule } from 'src/engine/core-modules/twenty-config/twenty-config.module';
 import { WorkspaceEntity } from 'src/engine/core-modules/workspace/workspace.entity';
-import { DataSourceModule } from 'src/engine/metadata-modules/data-source/data-source.module';
 import { WorkspaceManyOrAllFlatEntityMapsCacheModule } from 'src/engine/metadata-modules/flat-entity/services/workspace-many-or-all-flat-entity-maps-cache.module';
 import { ObjectMetadataEntity } from 'src/engine/metadata-modules/object-metadata/object-metadata.entity';
 import { PermissionsModule } from 'src/engine/metadata-modules/permissions/permissions.module';
@@ -23,7 +22,6 @@ import { WorkspaceCacheModule } from 'src/engine/workspace-cache/workspace-cache
       RoleTargetEntity,
       WorkspaceEntity,
     ]),
-    DataSourceModule,
     WorkspaceCacheStorageModule,
     WorkspaceManyOrAllFlatEntityMapsCacheModule,
     PermissionsModule,