feat(audiobooks): expose audiobooks/podcasts library types in admin UI

Adds 'Audiobooks' and 'Podcasts' options to the library-type dropdown
in the admin libraries page so operators can flag a folder as an
audiobook or podcast library. Extends contentLevelsForType() so the
admin UI's downstream filtering treats those types correctly
(audiobook -> ['audiobook'], podcasts -> ['podcast',
'podcast_episode']).

Backend scanner branches for these types were already wired in
sub-plan 2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/pages/AdminLibraries.tsx

@@ -1983,6 +1983,10 @@ function contentLevelsForType(libraryType: string): string[] {
       return ["movie"];
     case "mixed":
       return ["movie", "series", "season", "episode"];
+    case "audiobooks":
+      return ["audiobook"];
+    case "podcasts":
+      return ["podcast", "podcast_episode"];
     default:
       return [];
   }
@@ -2395,6 +2399,8 @@ function LibraryForm({
               <SelectItem value="movies">Movies</SelectItem>
               <SelectItem value="series">Series</SelectItem>
               <SelectItem value="mixed">Mixed</SelectItem>
+              <SelectItem value="audiobooks">Audiobooks</SelectItem>
+              <SelectItem value="podcasts">Podcasts</SelectItem>
             </SelectContent>
           </Select>
         </div>