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/database/commands/database-command.module.ts

@@ -28,7 +28,6 @@ import { TwentyConfigModule } from 'src/engine/core-modules/twenty-config/twenty
 import { UpgradeModule } from 'src/engine/core-modules/upgrade/upgrade.module';
 import { WorkspaceEntity } from 'src/engine/core-modules/workspace/workspace.entity';
 import { WorkspaceModule } from 'src/engine/core-modules/workspace/workspace.module';
-import { DataSourceModule } from 'src/engine/metadata-modules/data-source/data-source.module';
 import { FieldMetadataModule } from 'src/engine/metadata-modules/field-metadata/field-metadata.module';
 import { ObjectMetadataModule } from 'src/engine/metadata-modules/object-metadata/object-metadata.module';
 import { TrashCleanupModule } from 'src/engine/trash-cleanup/trash-cleanup.module';
@@ -62,7 +61,6 @@ import { AutomatedTriggerModule } from 'src/modules/workflow/workflow-trigger/au
     ObjectMetadataModule,
     DevSeederModule,
     WorkspaceManagerModule,
-    DataSourceModule,
     WorkspaceCacheStorageModule,
     ApiKeyModule,
     FeatureFlagModule,