refactor: remove podcast support

Podcasts shipped switched off in every release and are now removed from the server,
web client and shared types. Migration 0132 drops the podcast tables and the media-type,
library-type and folder-role columns. Released iOS and Android builds read the removed
response fields as optional, so the API change stays compatible.

server/file-env.sh

@@ -66,7 +66,6 @@ load_file_env() {
     GITHUB_RELEASES_TOKEN \
     JWT_SECRET \
     MIGRATION_ENCRYPTION_KEY \
-    PODCAST_ENCRYPTION_KEY \
     READ_ALOUD_WORKER_TOKEN \
     POSTGRES_PASSWORD \
     SETUP_BOOTSTRAP_TOKEN