Add updated_at column to vfolders table and expose in schema/DTO

Add an `updated_at` timestamp column to the `vfolders` table so that vfolder modification times can be tracked accurately.

Currently the table only has `created_at` and `last_used`, causing ModelCard UI to show incorrect modification timestamps.

## Scope
- Alembic migration: add `updated_at` column (nullable, default NULL)
- Add column to `VFolderRow` model definition in `row.py`
- Expose in GraphQL `VirtualFolderNode` / `VirtualFolder` schemas
- Add to REST API response DTOs
- Update `ModelCard.parse_model()` fallback from `vfolder_row.created_at` to `vfolder_row.updated_at`

## Success Criteria
- [ ] Alembic migration adds nullable `updated_at` column with proper upgrade & downgrade
- [ ] Existing rows have NULL for `updated_at` (no fake timestamps)
- [ ] `VFolderRow.updated_at` field exists in model definition
- [ ] GraphQL `VirtualFolderNode` exposes `updated_at` field
- [ ] REST vfolder response DTO includes `updated_at`
- [ ] `ModelCard.parse_model()` uses `vfolder_row.updated_at` as fallback for `modified_at`
- [ ] pants check/lint passes for affected packages

JIRA Issue: BA-5503