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/core-modules/billing/billing.module.ts

@@ -42,7 +42,6 @@ import { UserWorkspaceEntity } from 'src/engine/core-modules/user-workspace/user
 import { WorkspaceEntity } from 'src/engine/core-modules/workspace/workspace.entity';
 import { AiBillingModule } from 'src/engine/metadata-modules/ai/ai-billing/ai-billing.module';
 import { AiModelsModule } from 'src/engine/metadata-modules/ai/ai-models/ai-models.module';
-import { DataSourceModule } from 'src/engine/metadata-modules/data-source/data-source.module';
 import { PermissionsModule } from 'src/engine/metadata-modules/permissions/permissions.module';
 
 @Module({
@@ -66,7 +65,6 @@ import { PermissionsModule } from 'src/engine/metadata-modules/permissions/permi
       UserWorkspaceEntity,
       FeatureFlagEntity,
     ]),
-    DataSourceModule,
     MetricsModule,
     EnterpriseModule,
     WorkspaceIteratorModule,