D9/D10: users never edit podcast episode metadata. Three gates:
- Generic edit endpoint (UpdateMediaItem) rejects source=
'podcast_episode' items with 400 — the next fetch refresh would
clobber any edit anyway, and the fetcher is the sole metadata writer.
- Write-to-file (MetadataWriter.WriteMetadataToFile) refuses podcast
episodes for the same reason (tag write-back for downloaded episodes
is a named Phase-4 item needing its own iTunes-tag mapping).
- The book-detail page hides the admin Edit affordance for
podcasts-type items (direct-URL visits), and the metadata-editor
modal's field list has no podcast fields by construction (fixed
templ field list) — episodes render read-only via /podcasts
surfaces.