## 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
