Apple Music's recently-played history endpoint only returns ISRC for full catalog "songs" resources. Tracks played from the user's personal library come back as "library-songs" and omit ISRC entirely, which silently degraded downstream metadata matching (e.g. ISRC-based lookups in transformers) for most personally-owned library plays. When a track is missing ISRC and is a library item linked to a catalog song (playParams.catalogId), make one additional catalog lookup to backfill it, mirroring the existing Spotify Source's enrichIsrc pattern. Result is cached (including negative caching) so repeat polls don't re-fetch. Storefront is resolved once and cached on the instance. Adds options.enrichIsrc (default true, ENV APPLEMUSIC_ENRICH_ISRC) to allow disabling the extra calls.