Body byte-identical to the original. Spotify proxy via registry,
iTunes/Deezer client shims wrap registry helpers,
_resolve_library_file_path, _attempt_download_with_candidates, and
missing_download_executor are injected via init() right after
_init_wishlist_failed where all three deps are already defined.
web_server.py: 35239 → 35063 (-176 lines).
