Add missing index on viewField.viewFieldGroupId (#19253)

## Summary

- Adds an index on `viewField.viewFieldGroupId` to fix a ~28s `DELETE
FROM core.viewFieldGroup WHERE workspaceId = $1` query observed in
production
- The slow query is triggered during workspace hard deletion
(`deleteWorkspaceSyncableMetadataEntities` in `workspace.service.ts`)
- Root cause: the FK constraint `viewField.viewFieldGroupId →
viewFieldGroup.id` with `ON DELETE SET NULL` forces PostgreSQL to
sequentially scan the entire `viewField` table for every deleted
`viewFieldGroup` row, because no index exists on the referencing column
- Uses `CREATE INDEX CONCURRENTLY` in the migration to avoid locking the
table on production

packages/twenty-server/src/engine/metadata-modules/view-field/entities/view-field.entity.ts

@@ -32,6 +32,7 @@ export type ViewFieldOverrides = {
 @Index('IDX_VIEW_FIELD_WORKSPACE_ID_VIEW_ID', ['workspaceId', 'viewId'])
 @Index('IDX_VIEW_FIELD_VIEW_ID', ['viewId'])
 @Index('IDX_VIEW_FIELD_FIELD_METADATA_ID', ['fieldMetadataId'])
+@Index('IDX_VIEW_FIELD_VIEW_FIELD_GROUP_ID', ['viewFieldGroupId'])
 @Index(
   'IDX_VIEW_FIELD_FIELD_METADATA_ID_VIEW_ID_UNIQUE',
   ['fieldMetadataId', 'viewId'],