Deduplicates the hand-rolled coalescing/caching maps, the Plex client cast,
the missing-serverId event guard and the progress-failure backoff, and drops
the MusicPlaybackService availability gate, which could never fail in
production.
Plex API returns Directory entries without type field, causing
TypeError crash in fromJson. Made fields nullable and migrated
callers to use mediaType enum and displayTitle getter instead
of raw string access.
edde746