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/metadata-modules/metadata-engine.module.ts

@@ -8,7 +8,6 @@ import { AiGenerateTextModule } from 'src/engine/metadata-modules/ai/ai-generate
 import { CalendarChannelMetadataModule } from 'src/engine/metadata-modules/calendar-channel/calendar-channel-metadata.module';
 import { ConnectedAccountMetadataModule } from 'src/engine/metadata-modules/connected-account/connected-account-metadata.module';
 import { CommandMenuItemModule } from 'src/engine/metadata-modules/command-menu-item/command-menu-item.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 { FlatEntityMapsGraphqlApiExceptionFilter } from 'src/engine/metadata-modules/flat-entity/filters/flat-entity-maps-graphql-api-exception.filter';
 import { FrontComponentModule } from 'src/engine/metadata-modules/front-component/front-component.module';
@@ -30,7 +29,6 @@ import { WorkspaceMetadataVersionModule } from 'src/engine/metadata-modules/work
 
 @Module({
   imports: [
-    DataSourceModule,
     FieldMetadataModule,
     FrontComponentModule,
     ObjectMetadataModule,
@@ -63,7 +61,6 @@ import { WorkspaceMetadataVersionModule } from 'src/engine/metadata-modules/work
     },
   ],
   exports: [
-    DataSourceModule,
     FieldMetadataModule,
     FrontComponentModule,
     ObjectMetadataModule,