feat(podcasts): edit segregation — feed metadata is fetcher-owned (task 1.8)

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.

templates/book_detail_templ.go

@@ -566,7 +566,7 @@ func BookDetail(user User, book handlers.MediaDetail, errorMessage string, tombs
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		if user.Role == "admin" {
+		if user.Role == "admin" && book.LibraryTypeName != "podcasts" {
 			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 59, "<button @click=\"showMetadataEditor()\" class=\"btn btn-secondary px-5 py-2.5\">")
 			if templ_7745c5c3_Err != nil {
 				return templ_7745c5c3_Err