Rename the in-root loader path response field from `file_path` to
`loader_path` (matching compute_loader_path), and persist it on
asset_references so the API reads it directly instead of re-resolving
against every registered model-folder base per request.
- add loader_path column (migration 0006) populated at scan/ingest from
the already-computed loader path
- response prefers the stored value, falling back to compute for rows
written before the column existed
