Moves the CLI's priority-name lookup into core/artwork so the native API and
UI can render "scan" instead of a bare priority number, replaces a brittle
string comparison with a presence check on the stored artwork, renders the
source path in the details section, drops the unused hash field, and aligns
date rendering with the rest of the dialog.
Extend the admin report spec to assert the full stored object and step
fields, and add a spec covering the queued object and lastAttemptFailed,
so a wrong JSON tag in toExplainDTO fails a test instead of shipping.
Exposes core/artwork.Explain over GET /api/artwork/explain?kind=&id=,
reporting stored trace and queue state without ever walking the chain
live, so the response can only leak history the server already has.
